ET-3400 ROM - Selecting a suitable EPROM or EEPROM

The original MCM6830A mask ROM is 1KB in size. the only EPROM I am aware of in 1KB size is the Intel 2708 EPROM and that is a hard to find part and awkward to program and operate because it requires +5V, -5V and +12V to operate and program. I happen to have two of them in my collection of EPROMs, one is installed in a DREAM 6802 I also own. I'm keeping them for that one day in the future I will be able to program them.

While the ET-3400 does have those voltages on the main board, they are not available at the IC12 ROM socket, so I would have to have jumper wires or an on-board voltage generator to operate. Messy. Furthermore there are very few modern EPROM programmers that can program 2708's and I don't have one myself. Lastly it's only 1KB and I want to be able to choose from at least two ROM options so I need 2KB minimum. So I'm striking that off as a potential replacement.

After that there is 2716 EPROM (2KB), 2732 EPROM (4K) in 24 pin size ICs, or a 2764 EPROM (8K) in 28 pin size. These are all still easy enough to obtain and program by most programmers. In some cases you would need the 12.5V programmable version to burn on some modern EPROM burners such as a 27C16. For the purpose of this project, 2KB or 4KB is plenty, so I can strike off the 2764 as an option.

In order to make the PCB small enough to fit in a 24 pin footprint I want to get away from 600mil wide EPROMs so that I have enough space for a surface mount IC, otherwise the EPROM will have to be carefully offset from being directly above the original socket. Also there is still a requirement to decode all four CS pins as I mentioned earlier so I have to make room for at least one 14 pin TTL logic IC for that.

I also considered EEPROMs in surface mount size and that should fit within the 24pin footprint. Choices are AT28C16 (2K 24 pin), AT28C64 (8K 28 pin). Both of these will do what I want and fit within the 24-pin 600mil footprint. I may have difficulty fitting in an SMD TTL logic chip, but I will try first. I can also use the back side of the PCB so that gives me some more real estate if I need it.

Shopping for those parts reveals that the SMD AT27C64 EEPROM is cheaper and more readily available than the AT28C16 EEPROM through on-line shopping sites such as Ali Express, so that's the one I am choosing. As far as I am aware ATMEL still make these chips so hopefully it won't be out-of-date before I get it off the ground.

this is the pinout of the AT28C64B EEPROM:


This has all the pins I need and them some that I don't need. I'll deal with those after I come up with a circuit diagram to handle the CS logic, or CE as it is labelled on this diagram.

next I start the circuit diagram...


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