Introductions
The XRX-III mod is a synchronous 500-baud wave shaper for the TRS-80 Model 1 computer. It greatly improved the reliability of reading data from a cassette tape. Note that this MOD is unnecessary on the latest ROM v1.3 as a fix was put in the software. Additionally, any mods for 750-baud and 1000-baud mods will not work with the XRX-III mod.
Here’s an article clip from the May 1979 Radio Shack MicroComputer Newsletter issue. The XRX-II was free with the Level-II upgrade. You could buy the mod for $3.50 if you wanted to install it yourself.
Installation
Installation consists of cutting one trance and connecting 6 wires. After removing the screw on the bottom, pull off the top cover of the TRS-80. Flip the keyboard, giving you access to where the XRX-III needs to be installed. It’s recommended to place the XRX-II PCB just below where the ” ? ” key is on the keyboard PCB and attach it with double-sided tape.
Using a sharp knife, cut the trace between Z23 pin 9 ,and Z4 pin 10.
Connect the red wire to +5v Z45 pin 10.
Connect the black wire to ground Z44 pin 8.
Connect the yellow wire to Z43 pin 9.
Connect the green wire to Z4 pin 10.
Connect the blue wire to Z44 pin 12.
Connect the violet wire to Z24 pin 9..
Recreating the XRX-III
RetroStack has recreated and posted the original design and gerber on his git repo.
Japanese TRS-80 Model 1
The Japanese version of the Model 1 has custom ROMS to accomadate Kana characters. This was based off the v1.2 ROM. This would mean that all Japanese Model 1 units would benefit from this mod. At the time of the Japan Model 1 release, this issue was already known by Radio Shack. Interestingly, the earlier models sold in Japan had a completely different mod for this. It consisted of 2 resistors and 2 film capacitors along with the cut trace, as shown below.
Later versions of the Japanese Model 1 had the XRX-III mode installed, but it’s interesting to see that there was an earlier version to the XRX-III mod.
Leave a Reply