I'm trying to replace a 27c256 ROM with a 62256 RAM, to be able to easily change the memory content. I thought this would be pretty straight forward. Not quite, it seems.
I made an simple experimental PCB which is build around the 66256 RAM chip. On one side you have the connection to the target device, of course I made sure I rewired pin 1 with pin 27 to make sure A14 is in the right place. On the other side I have a PIC MCU with a RS232 connection to a PC which takes care of the programming and verifying of the RAM chip. To start of simple I placed DIP switches betwheen the RAM chip and both sides (Target device and PIC) to make sure the two aren't interfering with each other. My idea is to replace this with some type of octal latches eventually.
After I programmed and verified the RAM chip using the PIC I isolate the 62256 from the PIC and open the DIP switches towards the target device. I make sure I keep the /WE pulled up and the /CE pulled down. The /OE is strobed by the target device just as it does when a regular ROM is connected. However the target device does not run the program.
This while the program works fine when burned to a ROM.
What am I missing/forgetting?