Thursday, March 15, 2012

Replica 1 Multi I/O Board Hardware Mod

Tonight I made a little hardware modification to the the Replica 1 Multi I/O board that had occurred to me as soon as I got it. A EEPROM on the board is writable as normal memory, making it easy to program. The downside is that a bug in a program could write to the EEPROM and corrupt it. I added a write enable jumper so I can make it read-only when desired.

Circuit Modification

I just needed to break the signal to the /WE (Erite Enable) pin of the EEPROM and run it to a jumper so it can be connected or disconnected from the signal driving it. I added a pullup resistor to drive it high when the jumper is removed so the EEPROM cannot be written to. When the jumper is installed it operates as read-write.

Write Enable Jumper on Prototype Area

I cut one track between two vias and ran them to a connector on the prototype area. I added the 3.3K pullup resistor there too.

Jumpers from Cut Track

As much as I hate to desecrate a clean board, the prototype area was put there for a reason, to let people modify the board.

In Operation

Incidently, I found that some 28C64 series EEPROMs won't work in the Multi I/O board because they don't support byte write mode. A Xicor 28C64A had this issue. Most 28C64 and 28C64B devices seem to support this write mode.

Back in the days (early 1980s) when I programmed code into EPROMs, I spent a lot of time waiting for the EPROMs to be erased in an ultraviolet light eraser and then programming them with a specialized programmer. At the time we thought it was a big advance over the earlier PROMs that could only be programmed once and then had to be thrown away. I sure could have used EEPROMs like this back then.

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