Nov 242013

I previously designed a small adapter board to allow a regular 2764 EPROM to be plugged into a TRS-80 Model 4 (Gate Array version only) in place of ROM chip U4, in order to use the new auto-booting feature of the FreHD hard disk emulator. As I posted earlier, the assembled board was a bit too tall to fit under the aluminum shield that is present on US Model 4 computers, and it also needed a couple of pull-up resistors added. I changed my board design to address these problems.

I’ve received my new PCBs from the manufacturer, and they have solved the height problem that I encountered with the first boards. I reduced the height of the assembled board by replacing the DIP sockets with individual socket pins made by Mill-Max. These socket pins required larger holes in the PCB, and I added a couple of necessary pull-up resistors while I was at it.

The socket pins allow the ROM and EPROM to sit almost flush with the PCB when plugged in. They are a tight fit for the IC pins, so extra care is necessary to avoid bending pins when plugging in the chips. I stuck some vinyl equipment feet on the bottom of the board to keep it from popping out of the socket if the left side of the board is pressed.

All of the board files are available on GitLab:

I have also shared the design at OSH Park so that others may order bare boards:

Order bare PCBs at OSH Park

I wanted to be able to select between the original ROM and new EPROM without opening the computer’s case. I mounted a small SPDT toggle switch on the aluminum shield so that it could be flipped through one of the vent slots, and connected it to the ROM/EPROM select jumper on my board. I had to do some minor filing of the vent slot

  6 Responses to “Version 2 of TRS-80 Model 4 ROM Adapter”

  1. Avatar

    Hi Mark:
    You say this is only for the Model 4 Gate Array version only. Doesn’t the 4 Non-Gate-Array system use a similar ROM, or what (in very broad terms) would have to be changed to make this work on a 4D?

    Which of the Mill-Max pins did you use?


    • Hi. I do not have access to or experience with the earlier non-gate-array Model 4, so I have no idea whether this adapter will work with it. For the 4D, I do not know whether they were all derived from the gate-arrat version or not. If I ever do hear of anybody successfully using this board in a non-gate-array machine, I’ll try to remember to update this page with that news. If it would work electrically but not fit mechanically, then maybe somebody could modify my design? All of the design files are available if somebody would like to adapt it for other uses.

      For the two IC sockets, I used Mill-Max 0552-1-15-01-11-27-10-0 socket headers, available from Digi-Key as part number ED5037-ND.

      For the male pis on the bottom, I used Mill-Max 350-10-164-00-006000 pin headers, available from Digi-Key as part number ED6864-ND.

      • Avatar

        It only took me like 4 years to get to this point, but I can confirm this works with the Model 4D. I got this to play around with writing a custom ROM for a TRS-80 that was found by a friend of mine. Now that I’ve proven I can read and write the ROM successfully I can start playing with doing something more interesting.

        The hardest part was finding a good ROM dump to load onto the EEPROM. in the end I just built a custom reader using an arduino and the 2364 pinout to dump my original ROM. th nice thing was I was able to use the reader to verify that the EEPROM was correctly wired up by using the adapter in the reader to read the written EEPROM.

        Anyway thanks and good luck.

        • Oh, wow, it took me forever to notice your post in the moderation queue and approve it. I’m really sorry about that! I’m happy to hear that my board workd in the Model 4D.

    • By the way: It has been long enough since I worked on this that I do not remember whether I was aware of some reason that it would not work with the NGA model, or if I was just being conservative since I didn’t have an NGA to try it out with.

  2. Avatar

    Non-gate array Model 4’s use a standard 2716 EPROM for ROM C, the one that holds the FreHD auto-booting code. If you wanted to switch between that and the stock ROM, you should be able to just bank switch a 2732 EPROM that is burned with the stock 2KB in half, and the FreHD modified 2KB in the other half.

    Here’s an example of how bank switched is wired up,

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