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:
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