Thursday, June 22, 2017

Using SD Cards with the TRS-80 Model 100

Most retro-computers have some form of SD or Compact Flash storage options available these days. The TRS-80 Model 100, Tandy 102 and Tandy 200 were no exception, and these micros had such a device not so long ago, the NADSBox. Most unfortunately it appears the NADSBox can no longer be sourced. So what to do now? Well build my own, very cut-down version I suppose is the obvious answer.

Loading files into the Model 100

As an 80's portable computer with limited native storage, the Model 100 came with some nice features for file transfers built right into the standard ROM and hardware. These features allowed for connecting tape drives, a modem, or better for modern needs, a serial RS232 port for computer to computer transfers. The built in TELCOM package allows for some easy ways to transfers files, and nicer still, the built in BASIC also makes it quite easy to load programs directly from the serial port. So all that needs to be done (of course it'll all get much harder later) is to tap into the 100s native abilities and throw some files back and forth over the serial lines.

To start some hardware is needed for some initial testing, I used a Arduino UNO, an SD Card Shield and a MAX232 IC. The MAX232 is required to convert RS232 level signals to TTL serial suitable for interfacing with the Arduino board. There is not much to it, all that needs to be done is to connect the Arduinos TTL lines to the serial port on the Model 100 via the MAX232. There are a load of examples for wiring up the MAX232, this guide at StackExchange is the one I quickly referred to.

On the software side I wrote up a quick program for the Arduino to send some files from an SD card to the Model 100 (Note the SD and SP Atduino libraries are required). The Ardunio waits for the Model 100 to request a transfer and will then upload a file. On the Model 100 side of things a couple of lines of BASIC is need to first OPEN the COM port, send a request, then download / run the files sent by the Arduino.

In the tests I found a baud rate of 1200 is about the best that can be expected when loading BASIC files via the serial connection. The Model 100 tokenises the text being loaded directly into BASIC, and faster baud rates give errors during this process, I guess there is only so much a 2.4 MHz 80C85 CPU can handle.

Content that the serial SD card loading was quite successful, I'll move along with this project further and see how far I can get with it.
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