Tuesday, June 25, 2024

ZXIO V2 Experimenters Kit Out Now

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ZXIO V2 IO 8255A Experimenters Kit Parts

After working extensively on the ZXIO Blog series, I aimed to create a memory-mapped I/O board ideal for experimenting with ZX BASIC, as the ZX81 lacks the ability to use IRQ-mapped devices without resorting to machine code routines. The end result is the ZXIO V2, which is based on the use of an 8255A Programmable Peripheral Interface IC. 

Apparently, I wasn't the only one wishing for such a device. The blog series was extremely popular, and the demand for an easy-to-use I/O board for the ZX81 was high. After additional development and testing, I'm finally releasing the kit.

ZXIO V2 IO 8255A Experimenters Kit Complete
A Built ZX81 ZXIO V2 Experimenters Kit

What's in the Kit?

  • ZXIO V2 PCB and parts: The main component of the kit and brains of the operation.
  • ZX BUS Expander PCB and parts: A ZX81 BUS extension card, the ZXIO or other cards of similar design can be attached to this board.
  • ZX IO LED Board and parts: A unique expansion board designed to be mounted directly to the ZXIO V2 expansion headers. 
  • ZXIO Breakout Board and parts: Designed to be used in conjunction with a breadboard for experimentation and prototyping. (Breadboard not included). 
  • IDC ribbon Cable.
The kits contain everything needed to build the base ZXIO V2 board, including an LED board and a breakout board. All parts are brand new, with the exception of the 8255A IC, which is a reclaimed component. This decision was made to manage costs effectively; sourcing new versions of this IC would significantly increase the kit price, potentially doubling it.

Support and Future Projects

As with the ZX-Key Keyboard before, the ZXIO V2 has a dedicated support page where updates and detailed information about the device will be posted. This page will include comprehensive build instructions, troubleshooting tips, and other relevant details to assist users in assembling and using their kits.

After the initial release, I'm planning additional posts and new projects centred around the ZXIO V2 and its usage.

Where to get a Kit?

Kits will be available at Sell My Retro, at the time of writing priced at $98 USD plus shipping.

Hope all will enjoy building abd experimenting with this ZX81 kit, and I'd love to hear about experiments and / or even host some ideas on this site. Enjoy.

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Saturday, May 25, 2024

Cronosoft Releases: Minoss Knossoss for the ZX81 on Tape

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Cronosoft has announced the release of Minoss Knossoss for the ZX81 on Tape. You can now dive deep into the legendary labyrinth and embark on an unforgettable adventure that brings the rich mythology of ancient Minoan Crete to life on your ZX81.

ZX81 Game: Minoss Knossoss Tape Cover Art
Minoss Knossoss the ZX81 game Released on Tape by Cronosoft 

For any great release you need exceptional cover art: I produced the base artwork for the cover of the physical release, featuring a menacing Minotaur alluding to the horrors that await the adventurous archaeologist. With Simon of Cronosoft completing the stunning package design and putting the game through rigorous testing on real hardware, ensuring smooth loading from the tapes.

So now there's no excuse not to experience the thrill of the labyrinth and the beauty of retro gaming with Minoss Knossoss on your ZX81. Stay tuned for the official release date and prepare for an epic journey! 

Uncover a Copy of Minoss Knossoss

ZX81 Version

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Monday, April 08, 2024

ZX81 Game: Minoss Knossoss

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ZX81 Game Minoss Knossoss, Screen shot  of Minoss Knossoss Title / attract screen

Having had Tut-Tut successfully ported to numerous 8-bit systems, it feels fitting to elevate the gaming experience by launching a sequel to the original tomb-raiding puzzle adventure in the form of 'Minoss Knossoss'.

Minoan Archaeology with the ZX81

Life on the ancient history speaking circuit had grown dull. Tired of recounting your successes at the tombs of Tut-Tut, you yearned for the exciting life of a practical archaeologist. Now, in 1924, after a month's journey by tramp steamers, you arrived in Crete at the Palace of Knossos.

Initially finding little, concrete leads began emerging from your excavations, presenting opportunities too good to pass up. Ancient tales of curses and the fabled Minotaur, along with the legendary workshops of Daedalus, beckon to the grand adventurer.

Here in Crete, where the labyrinth of the Minotaur lies, with mysteries and dangers aplenty, you'll cement your reputation as the greatest archaeologist of all time, or die trying.

ZX81 Game Minoss Knossoss, Screen shot of level 1 "kephala Naos"
Can you find your way into the labyrinths beneath Knossos?

Playing the Game

Minoss Knossoss is one part puzzle, two parts arcade action. The game comprises 20 levels, with the final level being the hidden workshops of the mythical craftsman Daedalus; accessible only to those who have mastered the labyrinths (accumulated 2500 points).

Collect gems, amulets, bracelets, keys, and hourglasses to earn points. Amulets freeze the player, while bracelets halt creatures. Hourglasses award you extra time to complete a level. Completing a level requires the player to collect keys, open doors, and move blocks before finding exits to lower labyrinthine levels, all while keeping an eye out for King Minos's mythical guardians.

Labyrinth Guardians

Strategically utilise hidden crevices to evade enemies, capitalising on the predatory instincts of Harpies and Minotaurs to distract them while you unravel the puzzles.

  • Serpents: Massive snakes roam the levels. They are timid, and while they won't actively hunt you down, be cautious—cornering them may provoke a strike.
  • Harpies: With the body of a bird and the visage of a human, these winged creatures embody an avian ferocity and supernatural penchant for vengeance.
  • Minotaurs: Renowned as the ultimate terror within the labyrinth, these formidable creatures relentlessly pursue their prey, turning the maze into a deadly game of survival.


  • Keys: ‘O’ left, ‘P’ right, ‘Q’ up, ‘A’ down, 'F' to pause and ‘R’ to reset the level (at a cost).
  • Joystick / Gamepad: A Kempston standard Joystick addaptor is supported.


  • AY Sound is supported via ZON-X sound cards and compatibles such as the ZXPand+

The Making of Minoss Knossoss

Minoss Knossoss stands as the direct sequel to Tut-Tut, a game I originally crafted for Paleotronic Magazine and the Sinclair ZX Spectrum. Following its initial release, Tut-Tut underwent widespread porting to various 8-bit platforms by a diverse array of developers. Notable among them are Dave Curran for the PET, myself and Dave for the Vic20, Sheila Dixon's adaptations for the RC2014 and MSX machines, and George Beckett's rendition for the Jupiter Ace.

Having played a role in each iteration of Tut-Tut's porting process, we seized the opportunity to introduce numerous enhancements with each development cycle. However, there inevitably comes a point where further improvements reach a natural limit, paving the way for a full-fledged sequel.

ZX81 Game Minoss Knossoss,  Screen shot of level  "Teucers Bow"
Deeper into the Labyrinth, Level Teucers Bow

In essence, the gameplay of Minoss Knossoss will feel familiar to fans of its predecessor. However, the sequel introduces additional monster types, including Serpents, Harpies (with movement similar to the mummies from Tut-Tut), and, of course, Minotaurs, each contributing an extra layer of challenge. And what would be the point of new monster types without the ability to have more of them on screen at one time? Furthermore, players will encounter a new item type in the form of hourglasses, which provide valuable extra time to navigate through certain levels.

Among the less conspicuous enhancements are newfound abilities to strategically trap monsters, sparing players from potentially frustrating level restarts. Also note that, players can now enjoy the long-awaited feature of pausing the game, particularly notable feature for ZX81 users. Additionally, the playing area has been expanded, offering a more immersive gaming experience.

Behind the scenes, I've implemented enhancements to optimize how levels are stored, Refined game timing to synchronize directly with the clock cycles of the ZX81/Z80, resulting in smoother gameplay. And, of course, I've finally integrated joystick support and rudimentary sound for those fortunate enough to be using ZON-X sound cards and Kempston Joystick adapters (or emulators).

Regrettably, the one feature omitted is the level code input option, allowing players to skip completed levels. Unfortunately, due to space constraints, this feature couldn't be included. However, rest assured that it will make a comeback in future ports for other 8-bit machines—just a hint of things to come.

With all of these features and more, I hope you enjoy Minoss Knossoss.

Uncover a Copy of Minoss Knossoss

ZX81 Version

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Sunday, January 07, 2024

ZXIO Interface for the ZX81: Part 6

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ZX81 ZXIO TalkBot interface and ZXIO Interface
ZXIO-TalkBot and ZXIO V2 (with minor revisions)


Alright, we've got our reliable ZX81 all decked out with the ZXIO V2. Now, let's kick things up a notch - how about introducing an SP0256 (the original retro) speech chip to the party? This little marvel can transform pre-defined sounds (allophones) into speech, bringing a whole new layer of excitement to our setup.

Prior to designing the Talkbot, I made a slight modification to the ZXIO V2 (now V2.1 I guess) board. I introduced a new 4-pin header on the left side of the board, incorporating the NMI, Reset and Clock (foreshadowing)  signals from the ZX81, along with a Gnd line. This addition was made with the anticipation that the extra signals would be beneficial for future expansions. Moreover, the header enhances stability when connecting more extended expansion cards directly to the front of the ZXIO interface.

Of course it's well worth noting that it's quite possible to build an entirely separate / standalone interface board for the SP0256-AL2. When designing the ZXIO-Talkbot, I consulted a number of designs as a reference before planning the ZXIO plugin card. I highly recommend 'How to Make Your Computer Talk' by Steven J. Veltri as an excellent starting point. His book covers interfaces for a number of 80s home computers including the ZX81, along side in depth details on how the SP0256-AL2 itself works.

How to Make Your Computer Talk: T/S 1000, ZX80, and ZX81 Speech Circuit Schematic

The interface circuit from 'How to Make Your Computer Talk' is depicted above. Although as it's designed for Machine Code programs, it is not accessible to BASIC due to the absence of Memory Mapping. However, the fundamental design can be readily adapted for use with the ZXIO expansion card.

In the ZXIO-Talkbot, all addressing and data lines are interconnected with the 8255A on the ZXIO V2 interface card. Consequently, their control is handled in a manner similar to the earlier experiments where we employed the ZXIO V2 to manage an HD44780 LCD board. 

Unlike the 'How to Make Your Computer Talk' board, the clock signal is not produced by a separate 3.12MHz crystal; instead, we utilise the ZX81's own oscillator, which operates at 3.25 MHz—sufficiently close. Additionally, a separate RESET signal is not required; this line is directly connected to the ZX81's RESET. While, in future versions, it could be managed by the 8255A, such a configuration might be considered overkill.

ZXIO TalkBot Schematic, Featuring the SP0256-AL2 Speech Chip
ZXIO TalkBot Schematic, Featuring the SP0256-AL2 Speech Chip

The only other notable difference is the removal of the op-amp. Instead, I've provisioned a 3.5mm headphone jack that's easy enough to connect to powered speakers for all your amplification needs. (Note that there is also a direct connector that goes to my ZonZX-81 sound card.)

Talking the TalkBot

The SP0256 is a speech synthesis chip designed to convert phonetic information into audible speech. It operates by receiving allophone codes, which represent specific variants of a phoneme occurring in particular linguistic contexts. Allophones, unlike phonemes, are concrete variations of sound within a language.

In short, this means that you can't simply give the TalkBot a word and have it 'say' it correctly. Instead, you need to supply it with an allophone list that will hopefully construct a word from its sound samples. Below is a list of allophones, their sound descriptions, and suggested timings that each should be allowed to run.

Unfortunately, the timings are not so useful in ZX81 BASIC, as the instructions take more time to process than desired. Feeding the Talkbot the allophones and ignoring time signatures works sufficiently well. Of course, there's nothing stopping us from addressing the Talkbot/SP0256 later in Assembly for a little more accuracy.

ZXIO SP0256-AL2 Allophones Reference for the ZX81
SP0256-AL2 Allophones Reference

To transmit data to the ZXIO-TalkBot interface we must first configure Port A and Port B on the 8255A IC for output mode. This can be done by POKEing the control register at address 49151 with a value of 128. Once complete, we can begin transmitting control codes. 

The Enable line is first set low on Port B at address 49149, 0, while the allophone control codes are transmitted through Port A at address 49148, 'code'. Then Enable line 1 on port B is set high at 49149, 1. After that, the Enable line is brought low again.

ZXIO-TalkBot and ZXIO V2 interface cards (plus ZX Minstel ZXpand & ZonZX-81)

LCD and TalkBot

The fun doesn't end there; after all, the ZXIO is designed to allow multiple interfaces to be attached simultaneously. As such, we can reuse the LCD interface from the previous project in Part 5, and have it write out what we're asking the ZXIO-TalkBot to say at the same time. For this, we only need to use the upper bits at address 49149 (Port B) to enable and disable its control lines.

ZX81 ZXIO V2 running the LCD and ZXIO-TalkBot Togther Demo 

For your complete audio-visual enjoyment, please play the video below to witness and hear the results. This is just a sample of what could be achieved; the ZXIO V2 Cards and expansions offer almost limitless possibilities. For instance, they could be employed to power a wide array of applications, even extending to interfacing with devices like an Arduino. The potential is expansive, and the video provides just a glimpse of the creative possibilities.

For the moment, that pretty much concludes this mini-series on the ZXIO and ZXIO V2 expansion cards. Further projects based on the board are in the pipeline, and if there's enough interest, I may decide to produce kits and/or complete units. Please let me know if you'd be interested in obtaining a board. If you haven't already, please read over all the related earlier (and future) articles listed below.

See all the other entries for this project:   Part 1Part 2Part 3Part 4Part 5 and Part 6.

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Wednesday, December 06, 2023

Tut-Tut: Egyptomania the RC2014 Way

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Sheila Dixon unwraps a spooktacular new version of Tut-Tut, the modern Retro classic, playfully bringing 'Egyptomania' to the premier home-built computer of the modern Retro era, the RC2014.

Tut-Tut Level Ruby Rhod playing on the RC2014
Tut-Tut Level Ruby Rhod playing on the RC2014

What's New is Old when Tomb Raiding

Why let the ancient microcomputers of the Pharaohs have all the fun when there are splendidly modern Z80 machines ready to embark on the Tut-Tut adventure. With the RC2014 debut, Sheila has seamlessly amalgamated elements from diverse game versions, infusing captivating sonic enhancements that add an extra layer of discovery.

Part of the discovery may entail constructing your own RC2014 and then crafting a suitable video card for this petite computer, either a TMS9918A video module designed for RC2014 or a TMSEMU graphics card designed for RC2014, required as notably a base RC2014 kit only provides serial out.

The TMS9918A video IC was a fixture in numerous 80s home computers, including MSX1, so fittingly a touch of ancient technology is required to play the new RC2014 version of Tut. The TMS9918A brings a very vibrant palette to the game, remenicant of the ZX Spectrum, yet with it's own unique touches and a level of colour variety that puts the Sinclair to shame, delivering a new visulal experiance to the all the levels.

Screen Shot of a Screen of a Level of Tut-Tut RC2014
Screen Shot of a Screen of a Level of Tut-Tut RC2014

Talking about levels, this edition incorporates both the freshest additions and timeless classics from the Vic20 release. What might not be immediately apparent is that, for the most part, they are constructed using the ZX81 level definitions. This peels back a layer of the game, restoring a slightly more claustrophobic ambience and an oddly menacing urgency to escape each tomb. Just like in the Vic20 release, level codes are provided, so you need not remained encased in a failed level forever however. 

One of the most exciting additions to the RC2014 version is the audio overhaul. Sheila has added a new score and sound effects thanks to an optional requirement of an AY sound module. (Another fun addition to the RC2014 to build). Perhaps the best way to judge is to hear and see the game in action in a short clip taken during the games deveopment.

In short, the RC2014 proves to be an outstanding addition to the Tut-Tut lineup, coupled with the added bonus of running the game on a system you've constructed yourself.

You can dig up a copy of Tut-Tut RC2014 over at Itch IO, and don't forget to check out all the other versions of the games listed below.

Raid the Pyramids for a Copy of TuT-TuT

RC2014 CPM Version for the TMS9918A Video Card

Commodore Vic-20

Commodore PET

ZX81 Versions

ZX Spectrum Versions

Jupiter Ace

TRS80 MC-10

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Friday, August 18, 2023

ZX81 Bus Extender: The Expansion Board Solution

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Adding expansion boards to the back of a ZX81 is always a slightly terrifying business, a fear compounded with each consecutive item tacked behind the little wedge.

The wobbly expansion of expansions is a near unsolvable problem when using off the (distant in time and space) shelf products, however, when building our own expansions there's really no need to suffer. What's the answer to the wibbly wobbly's? Why a decently designed Bus Extender board and case of course.

ZX81 Bus Extender with an Assortment of Add-ons

We're not embarking on a wheel-reinventing adventure here. The concept of a Bus Extender isn't a groundbreaking discovery; Over the past four decades, resourceful enthusiasts have crafted their versions for the ZX80 and 81. The only hiccup? Laying your hands on one. But behold the magic of our modern age: securing your very own Bus Extender is a breeze. A simple solution? Just place an order for a PCB at any of the numerous fabrication houses like JLCPCB, PCBWAY or AISLER. Now, of course, you'll need something to order, and lo and behold, we've got you covered with the ZX81 Bus Extender gerber files conveniently linked below. Your expansion dreams are just a click away!

Refer to http://www.zx81stuff.org.uk/zx81/hardware/MaplinExtender for details
Maplin Bus Extender Interface (available at one point in the UK)

The Extender boasts a well-thought-out design, strategically accommodating up to three expansion cards and further expansion possible behind (Think ZXpand or similar). Its layout incorporates a three row hole arrangement on the printed circuit board, with ZX81 BUS 1a duplicated twice, offering the flexibility to house either two-row expansion headers or card slots – you know, the same kind that snugly attach to your trusty ZX81. This Extender knows how to cater to your every expansion whim. You'll require at least one expansion header to attach the thing to a ZX81 or ZX80, and  additional headers of your own choosing for accomodate the expansion cards.

zx81 bus extender / expansion edge connetor
ZX81 Adventures 3x3 Bus Extender

Naturally, a bus extender isn't fully dressed without its cosy enclosure. Picture it nestled within a polished case, a snug retreat ensures seamless operation, cradling expansion cards of your creation. The case boasts an elegantly simple design, seamlessly complementing your ZX81's rear. Oh, and don't forget those rubber feet (12.5mm*12.5mm*3mm(L*W*H)) – they elevate both style and stability, and some M3 screws to hold it together.

zx81 bus extender / expansion edge connetor in 3d printed case
ZX81 Adventures 3x3 Bus Extender in Handy Dany Case with Cards

Project Files

If you find these files useful, flip me coffee or beer etc.

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Sunday, July 16, 2023

Tut-Tut: Tomb Raiding on the Vic-20

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Tut-Tut Concept Art

A New Version of a Modern Retro Classic

Given the resounding success of my little game of Tut-Tut on platforms such as ZX Spectrum, ZX81, Jupiter ACE, and the commendable port by Tynemouth Software's Dave Curran to the Commodore PET, it was an inevitable progression to broaden the game's horizons and introduce it to the illustrious Vic-20, the "Wonder Computer of the 1980s."

New to Tut-Tut? What's this Game About?

As the digging season of 1921 draws to a close in Egypt's Valley of the Kings, it becomes evident that your excavations have yielded disappointing results. Despite your ardent efforts, no traces of the fabled and elusive Pharaohs' tombs have been uncovered. However, as the final weeks approach, intriguing rumours begin to circulate, recounting chilling accounts of vengeful mummies exacting retribution upon local would-be tomb raiders.

These wild stories, though tinged with the supernatural and curses, cannot be ignored. As an Egyptologist and daring adventurer, you find yourself presented with concrete leads and irresistible opportunities. The allure of uncovering long-lost secrets and ancient treasures proves too tempting to resist, even in the face of ominous tales of wrathful spirits.

Tut-Tut Vic20 Screen Shots
Screen Shots from the Vic-20 version of TuT-TuT

Now, on the cusp of a new expedition, you stand poised to delve into the unknown depths of the Valley of the Kings, ready to face the mysteries and potential dangers that lie ahead. Will you emerge unscathed from the clutches of ancient curses, or will the vengeful spirits lurking in the shadows prove to be more than mere legends? The answers await, as you embark on a daring quest that will test your courage, knowledge, and perhaps even your beliefs.

Prepare to embark on a journey like no other as Tut-Tut takes you on an adventure through the treacherous Egyptian tombs, with its visually stunning colour scheme and a myriad of intricately woven graphic tile sets that bring the ancient world to life before your very eyes.

The Making of the Vic-20 Version  

Tut-Tut initially originated as a BASIC type-in game, designed specifically for the ZX Spectrum and published in Paleotronic Magazine. Further versions of the game for the ZX81, Enhanced 2020 Spectrum Edition and Jupiter Ace, the latest version was one ported to the Commodore PET by Dave Curran. All varieties have all kept the core structure and mechanics as outlined there.

After witnessing the success of Dave's PET version, I couldn't help but ponder the possibility of bringing Tut-Tut to the Vic-20. While considering the similarities between the two machines, I became aware of the numerous disparities that needed to be addressed, particularly in terms of screen real-estate and the Vic-20's unconventional memory management scheme. Luckily i raised the conversion idea with Dave, and he was up to a joint challenge, as I'm in no way sure I'd have got there without our joint efforts.

Screen Real-Estate and Luck by Design

Due to Tut-Tut's origin as a ZX BASIC game, one that need to run quickly, lead to the adoption of a 26x17 character grid for the game board layout (excluding score bars etc). The ZX Spectrum's character resolution is 32x24, constraining the board size to a smaller grid made for more time efficient code in the very orriginal ZX BASIC version. These dimensions sunsiquently stuck for all future conversions.

However, while the grid size was suitable for the ZX line, it posed a challenge when porting Tut-Tut to the Vic-20. Those familiar with the Vic-20 immediately noticed the issue: the Vic-20's screen width is only 22 characters wide (and 23 high). This presented a hurdle that needed to be addressed in order to adapt the game for the Vic-20.

A possible solution could have been to reconfigure the game's levels to fit within the Vic-20's 22x23 screen limitations. This would have been easily achievable code wise, but level design wise it would come at a cost to the overall feel and immersion. A particular issue, as the existing levels are heavily constructed around the games mechanics and already small(ish) screen size.

Tut-Tut Vic20 Screen Shots / PAL vs NTSC Geometry
PAL vs NTSC Geometry Comparison

As luck would have it, the Vic-20's screen sizes can be redefined to an extent. Values in memory location 36864 to 36867 can be POKED to set screen geometry and centring. The results of doing so yielded just the right amount screen width to fit the existing level structure.

Yet another obstacle presented itself: the disparities in screen geometry between PAL and NTSC standards. NTSC, in particular, occupies a wider physical width on a display (not pixels) compared to PAL. However, by a stroke of luck, the constraints imposed by the level width inadvertently resulted in an image that delicately brushes the very edge of an NTSC display. This serendipitous alignment allowed Tut-Tut to make the most of the available screen space on both PAL and NTSC systems.

Graphics Updates

The Vic-20 is perhaps renowned for its vibrant and colourful display, you'd think this would be directly comparable to the ZX Spectrum in terms of its rich palette. However, an intriguing aspect arises when considering the variation in colour appearance based on factors such as the age, make, and region (PAL/NTSC) of the Vic Chip utilised on any given machine. Then there is the screen fuzziness / blurriness, which again varies. 

The diversity in colour appearance on different Vic-20 machines prompted us to reevaluate tile design and colour choices for Tut-Tut. In the Spectrum version, the game employed various palettes that cycled based on the level number, creating a dynamic and ever-changing visual experience. However, for the Vic-20 version, we've opted for a different approach. Instead of cycling palettes, we focused on expanding the selection of available thematic tiles while exercising tighter control over the colour schemes. (A good approach, and If I hadn't mentioned it, would probably gone unnoticed.)

We're pretty confident we reached the required level of ancient Egyptian cinematic charm / ham.

Memory for Mummies?

Another challenge we faced was determining how much memory we could allocate to the game on the Vic-20. By default, the Vic-20 only had a modest 3k of onboard memory, which was insufficient to accommodate even the most streamlined versions of Tut. The issue was further compounded by the lack of a standardised expansion memory layout and location of that memory on a Vic-20. This variability made it difficult to rely on expanded memory for larger games, often leading to the reliance on cartridges for larger game titles.

Ultimately, we proceeded with the assumption that modern enthusiasts would likely possess a contemporary memory solution, such as the Penultimate+, offering a generous 32k of memory. This assumption gave us the confidence that the game could be released on cassette. Little did we know at the time of developing the game that Tut-Tut would eventually become a pack-in title on the Penultimate+2 Cart from TFW8b.com.

Pharaohs' (Pen)Ultimate Enhancements

The Vic-20 version of Tut-Tut comes packed with all 36 levels from the ZX Spectrum 2020 Edition. In addition, we have incorporated 6 exciting new levels (bringing the total to 40) that are carefully integrated throughout the game, providing fresh challenges and surprises.

We have also included the convenient pause option from the PET version, allowing intrepid Archaeologists to take much needed meal and rest breaks. Furthermore, one of the most highly requested features we have implemented is the inclusion of level codes: Enter specific codes to access the harder levels previously reached.

All very good reasons to call this Vic-20 version, the Ultimate Tut-Tut.

Playing Vic20 Tut-Tut with the Peultimate+2 Cartridge
Playing Tut-Tut Vic-20 as included on the Penultimate+2 Cart

Special Thanks

I Can't end this post without thanking Dave Curran for sharing in the creation of the Vic-20 conversion, he'd paved much of the ground work during his PET efforts.

Additionally, thanks very much to Rob Hull for including the game in the Penultimate+2 games lineup. Very happy indeed to have it listed alongside the other TFW8b titles.

Fancey Delving Deeper into the Vic-20 Conversion?

Further Readings  unraveling the mysteries of Tut-Tut Tomb's and the Vic-20 conversion are now available on the Illustrious Tynemouth Software Blog.

Raid the Pyramids for a Copy of TuT-TuT

Commodore Vic-20

Commodore PET

ZX81 Versions

ZX Spectrum Versions

Jupiter Ace

TRS80 MC-10

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Sunday, May 07, 2023

ZXIO Interface for the ZX81: Part 5

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IO Boards Comparison, Featuring Hitachi LCDs

Now that we have 2 differing IO boards, why not put them in a side by side comparison. I figured a good test of the board would involve a simple communications project, such as writing out to a character based HD44780 compatible LCD screen.

An HD44780 LCD panel is a character-based liquid crystal display that can display 16 characters per line and up to 2 lines of text. It is widely used in embedded systems and DIY projects because of its low cost, low power consumption, and ease of use. The HD44780 LCD panel is compatible with a wide range of microcontrollers and can be interfaced using a 4-bit or 8-bit command set. We really couldn't ask for a more amiable device for testing ZXIO board differences with.

HD44780 LCD Module Pin out 
2VCC+5 Volts
3VEEContrast adjustment 0V: High Contrast
4RSRegister Select 0: Command, 1:  Data
5R/WRead/Write 0: Write, 1: Read
6ENEnable. Falling edge triggered
7DB0Data Bit 0 (Not used in 4-bit Mode)
8DB1Data Bit 1 (Not used in 4-bit Mode)
9DB2Data Bit 2 (Not used in 4-bit Mode)
10DB3Data Bit 2 (Not used in 4-bit Mode)
11DB4Data Bit 4
12DB5Data Bit 5
13DB6Data Bit 6
14DB7Data Bit 7
15LED A+Anode Back light +  
16LED K-Cathode Back light -

Configuring for the ZXIO V1 Board

Connecting the LCD module to the ZXIO is a straightforward process. The output pins O0 to O3 of ZXIO should be paired with DB4 to DB7 on the LCD board to send data from ZX81 to the module. In addition, the output line 6 of ZXIO must be linked to Register Select, while the output line 7 should be connected to the Enable Pin.

The lines responsible for controlling the power to the LCD module and screen contrast are as follows: VSS is linked to the ground, VCC is linked to +5 volts, and VEE is connected to the ground through a variable resistor (across the +5v line). To adjust the screen brightness, connect the LED+ pin to +5 volts and the LED-pin to the ground through a 220 ohm resistor (resistor is optional as some of these Module clones have the required resistor built in).

ZXIO V1 to LCD Module
That's the hardware out of the way, the rest is all down to some BASIC programming on the ZX81 targeting the ZXIO and LCD display module.

HD44780 LCD Commands (Examples)
Code (HEX)
Code (DEC)
Command to LCD
0x011Clear the display screen
0x022Set to 4 Bit Mode
0x0e14Set Underline Cursor
The below program connects the HD44780 to the ZX81 / ZXIO V1 at address 16507. It initializes the HD44780 by sending a sequence of control codes to set the display mode, enable the display, clear the display, and set the cursor to the home position. To send each byte, it needs to be split into 2 * 4 bits and sent consecutively. The Enable line must be brought high and then low to signal each 4 bit segment sent.

Subsequently, the program transmits the message "HELLO FOUR BITS" to the HD44780 by encoding each character as its corresponding ASCII code. To achieve this, the ZX81 Characters  should to be converted to their ASCII counterparts. As previously mentioned, each byte is then split into two 4-bit segments, with both the Enable and Register Select lines being set high. After transmitting each 4-bit segment, the Enable line is set low once again.

ZX81 Code to drive LCD in 4bit Mode

The program sends data to the LCD screen at a slow but satisfying speed. While this could be accelerated with code optimisation and pre-conversion of the ASCII text, I opted to maintain program similarity between the code directed at the V1 and V2 boards for a more effective side-by-side comparison (see next section).

Output from ZXIO V1 4Bit LCD Program

Configuring for the ZXIO V2 Board

Setting up the V2 interface involves a process similar to that of the V1 version, with the added advantage of utilising the entire 8-bit input lines available on the HD44780 controller board. On the ZXIO V2 board, Port A pins 0 to 7 (facilitated by the 8255A chip) are mapped to the Data pins on the LCD. While, the Register Select and Enable lines are mapped to Port B pins 6 and 7, respectively.

ZXIO V2 to LCD Module

As we're using the full 8-bit input, the configuration commands we need to send to the LCD interface vary slightly as we no longer need to put the device into 4-bit mode. (In both cases we're only using a very small subset of the available command set, just enough to get things moving along.) 

HD44780 LCD Commands (Examples)
Code (HEX)
Code (DEC)
Command to LCD
0x011Clear the display screen
0x0e14Set Underline Cursor
0x3856Set to 8 Bit Mode, Configure Display
To transmit data to the LCD interface using the V2 version, we must first configure Port A and Port B on the 8255A IC for output mode. This can be done by POKEing the control register at address 49151 with a value of 128. Once complete, we can begin transmitting control codes. The Enable line is set high via Port B pin (address 49149), while the control codes are transmitted through Port A (address 49148). After each code, the Enable lines is brought low.

As in the previous version, we first convert our message "HELLO EIGHT BITS" to ASCII before transmitting it to the LCD. We begin by setting the Enable and Register Select lines to high via Port B. Next, we send a character from our message string to Port A, and after transmission, set the Enable line back to low.

ZX81 Code to drive LCD in 8bit Mode

Using the the ZXIO V2 board, our "HELLO" message is send somewhat more speedily, though still managing a rather 80s sci-fi future computer message output speed (All very MU-TH-UR 6000: Look out Ripply!).

Conclusions Drawn??

The discussion above only scratches the surface of the potential applications for both the V1 and V2 ZXIO boards. Despite its simplicity, the LCD test highlights the greater versatility of the V2 board in the long run. Nonetheless, this does not detract from the ease of use of the V1 board. With a simple address change to mend the issues outlined in previous blog posts, the V1 board is an ideal choice for a wide range of hardware experiments.

That being said, the ZXIO V2 design offers more possibilities for exploration due to the presence of the 8255A PIO chip. Future blog posts in this IO series will delve deeper into these possibilities.

Waiting for more in the IO series? Take a read of:  Part 1Part 2Part 3Part 4Part 5 and Part 6.

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Sunday, April 16, 2023

ZXIO Interface for the ZX81: Part 4

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ZX81 ZXIO V2 Input Output Card with LCD Shield
ZXIO V2 Card with LED Expansion Board

Previously I built a simple Input / Output board for the ZX81, tested it, identified some self induced errors then hinted that it may be worth changing designs completely. Way back in Part 1 I'd already come to the conclusion that I really shouldn't make this a simple project, so of course I went back and stared things again.

Why Change  Now?

Didn't the last design work and only really require some minor addressing changes? Yes, and that would be a perfectly fine end to the IO project. Still I wished to take this further and build a more capable board. Note that the overall aim of building a relatively simple interface is still a primary goal.

The IO version One card has a minor limitation in that it can only support an 8-bit wide addressing, which may not be sufficient for more complex hardware that requires access to at least a partial 16-bit width address space to access control lines while still being able to send and receive 8 bits. There are several ways to address this issue, such as using 4-bit modes or using 7 bits for data and the remaining bit as a control line. However, the feasibility of these solutions depends on the specific interface requirements of the project.

One possible solution is to double the IO options by adding an extra set of latch and buffer ICs. However, this would increase the complexity of building the board, including routing and address decoding. Other options could involve employing a "standard" IO IC.

The 8255A, the IO Chip of Choice (This Time)

Three suitable IO ICs come to mind for our purposes: the Z80-PIO (Parallel Input/Output Interface), the 8255A-PPI (Programmable Peripheral Interface) and the W65C22N-VIA (Versatile Interface Adaptor). All three of these chips are period correct for the ZX81, in production and available of the shelf today (at least in 2023).

For Version 2 of the IO board I selected the 8255A, as it's pretty well documented, and as a bonus it made an appearance in a ZX81 IO board designed by A. Daykin for Maplin's 'Project Book 04' from 1983. With some modifications to the Addressing and IO configurations to make it more suitable for experimentation, the Maplin board can be made pretty well perfect for our needs.

Period Inspiration from Maplin Project Book 04 - 1983
The 8255A IC is a chip that serves as a programmable peripheral interface, allowing for parallel input and output. It has three ports: Port A, Port B, and Port C. Each port can be configured as either an input or an output. Port C can be split into upper and lower blocks, each with the option to be programmed as an input or output. The IC also has a control register that is used to set the mode of operation for each port. With these features, the 8255 IC can replicate and even expand upon all the functions that the first version of the ZXIO board was capable of achieving. It can actually do fair bit more, but we may explore that latter on in this series of posts. 

Maplin IN 

The Maplins design is appropriately simple, making it easy to connect the 8255A to the ZX81. Note that 4 contiguous address locations require mapping, this performed partly by the 8255A and then the supporting ICs. The 8255A chip has two address lines on pins 8 and 9, which are directly linked to the ZX81 address lines Al and AO. The 74 series ICs then handle the remaining address decoding, and enabling of the 8255A when pin 6 is set to logic level 0. All data lines from the ZX81 are directly connected to the 8255A, along with write and read signals. The reset line on the 8255A at pin 36 is tied to GND.

The 16 IO pins of the 8255A that make up Ports A and C are directly connected to pin headers at the edge of the Maplin board for external device connection. However, the pins of Port B are linked to IC5 and IC6, which buffer the outputs from the 8255A. In conjunction with a set of 4.7k resistors, this setup offers protection against overload. The purpose of this configuration is to drive potentially higher voltage equipment from Port B. A side affect of the buffering is to limit Port B to output only.

Each IO Port and a Control Port are Memory Address Decoded back to the ZX81, Specifically, Port A corresponds to memory address 16380, Port B corresponds to memory address 16381, Port C corresponds to memory address 16382, and the Control Port corresponds to memory address 16383. These addresses are located at the top of ZX81s 8 to 16k range where a copy of the ROM would normally be shadow mapped. Refer back to Part 2 in this series for details on address ranges and suggested uses.

** For additional details on the Maplin Board I'd recommend Allan Faulds blog page, where he builds up an original Maplin IO Board purchased in the 1980's. **


Although there aren't many modifications needed to transform the Maplin into a ZXIO V2, there are a few adjustments I would like to implement to enhance the design's practicality for contemporary experimentation.

The initial modification I made was to adjust the address mapping to span from 49148 to 49151. This will position the device at the upper end of the 40-48k memory segment, beyond the reach of numerous contemporary and historic memory expansion cards (not all, but many). 

I eliminated the buffering ICs from Port B, if buffering becomes necessary, we can always incorporate that back into external hardware. I also took the opportunity to ground output lines on Ports A to C via 4.7k resistors, this will prevent floating values on the lines when they're not connected to anything. Additionally the Reset line on the 8255A is now tied to the Z80 / ZX81's reset signal, inverted through spare NAND gates from the address decoding ICs.

ZXIO V2 Schematic.

The last functional modification consists of two headers. The first one is an IDC header resembling the ZXIO version 1 board, which includes the IO Lines, Ports A to C, Ground, and +5 Volts. This facilitates the use of IDC cables to connect to external breadboards or built-up external devices. Additionally, I added a female header in parallel, allowing for direct connection to plug-in boards, like the LED "hat" shown in the photo at the top of this post.

ZXIO V2 Test Board

Next Post?

This should mostly cover the essential hardware details of ZXIO V2. In my next blog entry, I plan to conduct a quick comparison between the old and new ZXIO boards. Although I am confident that V2 is a more versatile board, V1 remains a decent option for basic experimentation. Lets see, stay tuned for the next post.

Until then see all the other entries for this project:   Part 1Part 2Part 3Part 4Part 5 and Part 6.

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