TRS-80-Model I Jap50-E1

TRS-80 Model 1 (Rev HE11E011550 - Japanese) Replica - E1

This project is a faithful reimplementation of the mainboard of the iconic TRS-80 Model 1 (Revision HE11E011550) computer, redesigned and produced in Japan later in its existance. My own revision is designed to replicate the original system's functionality by using a PCB design 1-to-1 to the original, including components, interfaces, and even traces. The entire project is available under the MIT license.

WARNING: This project has not been confirmed to work yet. Use at your own risk.

<No photo available yet!>

NOTE: This board was reconstructed from photos and using an earlier board (Japanese-20 board) as reference. Therefore, the majority of traces are accurate, but a few (~18)might slightly deviate from the original design.

Project Details

Latest Files

In the "Latest" folder, you'll find the most up-to-date design files, including:

• Gerber files suitable for popular online PCB manufacturers like PCBWay and JLCPCB. Most manufacturers should be fine with either.
• A Bill of Materials (BOM) in both CSV and PDF formats.
• The full schematics of the replica which is 1:1 to the original board.

Implementation

The replica has been implemented using KiCAD 9. The KiCAD project files are included in this repository.

Assembly Instructions

For each step, check if there is a connection between pad 1, 8, 9, 16 on Z18 (all four corners of the dynamic RAM IC). That IC needs all 3 voltages with ground. If any of them are shorted, then you know what you did most recently. Finding the cause of the short should then be relatively easy.

1. (optional) Install sockets for all ICs.
2. Install all diodes. These diodes are hard to read and if you accidentally mix them up, you might end up with incorrect voltages on the rails. Also, make sure to orient them correctly. There should be a black band on one side of the glass diode. Match this with the line in front of the arrow of the diode on the PCB silkscreen. Installing first the shorter components will make sure you still can flip the board around and solder things easily.
3. Install all resistors. Resistors do not have an orientation. Skip the potentiometers for now.
4. Install all disc-like capacitors. Do not install the electrolytic capacitors yet! All other capacitors are rather small and orientation isn't important.
5. Install all transistors, including the ones with the heatsinks. The TIP29 should go into Q3 (metal side away from board towards the Q3 label) while the TIP32C should go into Q6 (lay flat with heatsink; bend legs wit needle-nose pliers to fit). Don't forget to add thermal paste between heatsink and transistors. Make sure to add the screws and nuts and that it fits tightly as the screws themself connect traces. Not having them properly installed will result in a non-functioning system. The smaller black transistors have one flat side. These should all face towards the bottom of the board towards all the ICs.
6. Install the crystal (no orientation), relay (match orientation on PCB), full-bridge rectifier (CR1; match "+" marking on component with PCB marking), all DIN connectors, both switches, keyboard connector (if bend, bend it upwards - towards the CPU to fit in the case), and potentiometers (only goes in one way).
7. Install all electrolytic capacitors. Make sure the orientation is correct! Modern capacitors mark the "-" side. On the PCB, the positive side is marked. Incorrect orientation may result in a small explosion (loud and smokey, but probably not dangerous).
8. Only install Z1 & Z2 to calibrate the power supply. Don't install any of the other ICs yet!!!
9. Get Multimeter ready in voltage mode. Connect power, turn on S1. If something starts to smell burned, turn S1 off and disconnect power.
10. Check voltage between pad 1 (-5V) and pad 16 (GND) on Z18. It should be around -5.1V to -5.2V. Since this is clamped with a zener diode, this should be pretty accurate. There is nothing that needs to be calibrated here. If the voltage goes beyond -5.3V, check R19 (may be really hot! careful!). If R19 is hot to the touch, your power supply is probably broken. (I've seen this on one power supply while another one didn't have that problem.)
11. Check voltage between pad 8 (+12V) and pad 16 (GND) on Z18. It should be around +12.0V. If it isn't, change the R23 potentiometer. If it is +/-0.5V off, then this is fine. But, try to get it as close to the desired voltage as possible.
12. (You need to calibrate +12V first before going to this next step!!!) Check voltage between pad 9 (+5V) and pad 16 (GND) on Z18. It should be around +5.0V. If it isn't, change the R17 potentiometer. If it is +/-0.5V off, then this is fine. But, also here, try to get it close to the desired voltage.
13. Re-check all the voltages after calibration, just to be sure.
14. Turn off S1 and disconnect power. Do this for each of the following steps: After each change, re-check the voltages at Z18 to find shorts quickly.
15. Start inserting the ICs. Focus on the 74LSxx ICs first. They are cheap and are easily recoverable in case there is a short somewhere and you start frying ICs. May be do 2-4 chips at a time and re-check the voltages at Z18. Do this systematically from left to right and top to bottom, so you know which you inserted most recently. If one of the voltages changes significantly at Z18 (+/- 2V and more), then you probably have a short with the most recent ICs. Do not insert the CPU, ROMs, character generator, and RAMs (static (2114) and dynamic ones (4116)) for now. They are expensive to replace if fried.
16. Insert the 74C00 IC for the video sync. This IC is difficult to come by and somewhat expensive. If all voltages are good, then they are good to install.
17. Insert the 7404 IC for the clock. This IC is also relatively cheap, but they are harder to come by. If all voltages are good, then they are good to install.
18. Connect jumpers JP1 and JP2 to set it up for ASCII Level II ROMs.
19. Connect jumpers JP4 from C to 2 and JP5 from C to 2. This will fix the character generator installed in Z38 to be the only character generator used. If you install the Kana one, then you need to setup JP4 from C to 1 and JP5 from C to 1. There is even a third option, connecting JP4 from C to 1 and JP5 from C to 2, which let's you choose the character generator using IO port 255 and bit 7 (last bit).
20. Next, we need to configure the video mode as it supports both NTSC and PAL. For NTSC, set JP6, JP7, JP8, JP10 from C to 1. For PAL, set them all from C to 2.
21. Last jumper is for configuring the video output to be used with the right monitor. If you are using the original Model 1 monitor, you need to connect JP9 as it will provide +5V on the video connector to the monitor, which it needs. All other monitors do not need this and you can leave it disconnected.
22. Now, we still have to install a jumper across the board. See the jumper installed below (the photo is from an earlier revision, but very similar):

23. Connect to a monitor. Use a CRT, if possible, as video calibration is easier. LCD works too, but the reaction time is slow and requires a longer wait time after each change.
24. After turning on monitor and S1, you should see white blocks on a black background. No need to calibrate a potentiometer on this revision as this is done by the counter ICs automatically.
25. (Don't forget to turn off system!) Insert the static video RAM (2114). These ICs only require +5V.
26. Insert the dynamic RAM (4116). These ICs require all three voltages.
27. Insert CPU in Z48. Not much should have changed from the video calibration up until now. The next step will change that.
28. Insert character generator in either Z38 (ASCII) or Z37 (Kana) - or both. Instead of white blocks, you should see (random) characters. The system doesn't clear the video memory yet since no ROM is installed.
29. Install ROM. This should be a Level II. You can also burn the diag ROM on a 2k ROM to test the system. The diag ROM should work for all revisions of the TRS-80 Model 1.
30. At this point, you should see a prompt showing "Memory Size?" that keeps blinking (if it is the normal system ROM). The reason for the blinking is that the keyboard is not yet connected and the system recognizes keypresses. Install the keyboard with the IDC cable (if you installed a header).
31. Now, you should be able to use the system. Try to hold the left SHIFT key and press the right arrow key and the text size should increase, switching the video modes. (It is OK when some characters are skipped. That is normal.) The CLEAR key should reset it to the original video mode and clear the screen.
32. Try running a "Hello World!" program: 10 PRINT "Hello World!" (Return) 20 GOTO 10 (Return) RUN (Return). You can stop with the BREAK key.

Ordering Instructions

When ordering the board from a PCB manufacturer, you can select the following to get a more faithful version of the board:

Dimension: 407mm x 184mm
Layers: 2
Base Material: FR-4 TG 135-140 (and up)
PCB Thickness: 1.6mm
PCB Color: Green
Silkscreen Color: White
Via Covering: Untented
Surface Finish: Lead Free HASL
Outer Copper Weight: 1oz
Gold Fingers / Card Edge: yes (chamfer: 30 degrees)
Castellated Holes: no

Additionally, supply the following comment for the manufacturer:

Card edge is only on the upper right. The bottom right is just a connector and does not need to be chamfered.

There are some missing pads on the top layer. That is correct. Please leave them as-is.

This addresses recurring questions from some of the interesting design choices that were used on the original board which was replicated.

Bill of Materials (BOM)

Below is a list of materials needed to assemble a complete system.

RetroStack Libraries

To work with this KiCAD project, you'll need the RetroStack libraries for KiCAD. Please follow this link to access and install these libraries.

Contributions

A big thank you to Howard Bowe for donating the Japanese Model 1 (Jap-20), and to Leslie Ayling for providing detailed photographs of his original Jap-50 board. Their contributions made it possible to reverse engineer and create such a detailed and accurate replica.

Main TRS-80 Model 1 Repository

For additional resources related to the TRS-80 Model 1, be sure to check out the central page for the TRS-80 Model 1 on RetroStack.

Support this Project

RetroStack is passionate about exploring and preserving the legacy of older computer systems. My work involves creating detailed documentation and videos to share the knowledge. I am also dedicated to reviving these classic systems by reimplementing them and offering replacement parts at no cost. If you're keen on supporting this unique project, I invite you to visit my Patreon page. Your support would be immensely valuable and greatly appreciated!

License

This project is licensed under the MIT License - see the LICENSE file for details.