M62 – Simple RAM/ROM expansion card

This simple RAM/ROM expansion card allows the edition of 1 or 2 ROM/RAM chips onto the M62 system.

The configuration of the pictured card is 16K RAM and 16K NVRAM, the switches at the right allow the banks used to be selected. This one is configured to have the NVRAM at BANK_2 ($8000-$BFFF) and the RAM at BANK_3 ($C000-FFFF).

There is no decoding circuitry needed on this card as each chip takes up an entire bank and there is already decoded on the CPU card.

The diagram above shows the connections of a RAM chip to attach to the M62 bus, there is a set of jumpers to allow to selection of the bank to use. The pinout used it for a 32K RAM (62256) but since we are only using 16K we tie all address lines above A13 to 0V (in this case only A14 applies).

For the board pictured, I just duplicated this wiring, once for each chip. The jumper on the left of the NVRAM chip allows me to tie A14 high or low (allowing me to select which half of the chip to use) and the jumper on the right side allows me to disconnect the WE line which selects it as read only.

If you are using a ROM, then the WE will not be available on the chip, or if using a FlashROM, you can just tie WE to +5V if you have no intention of writing to the device.

A NVRAM will usually have the exact same pinout at the equivalent RAM chip.