10/10/23 Update

A project log for 1802 MemberChip Card

A retro-microcomputer in a micro-package. Learn from the ground up, just like the pioneers. Has monitor editor assembler BASIC FORTH LISP...

lee-hartLee Hart 10/10/2023 at 18:570 Comments

For those that already have a MemberChip Card rev.A, here are the changes to convert it to rev.B, to expand the ROM from 32k to 64k:

The change is pretty simple; just three pins! Compare it to the rev.A schematic.The EPROM is changed from a 27C256 (32k) to a 27C512 (64k). The 1802 reads the lower 32k of the EPROM normally when A15=0 and /MRD=0 (a normal memory read cycle).

But if the 1802 writes to the lower 32k, it sets A15=0, /MWR=0 and /MRD=1. /MWR=0 chip-selects the ROM, A15=0 enables it to read, and /MRD=1 sets ROM address bit A15 high; so it reads a byte from the upper 32k.

We don't want both the 1802 and ROM to put data on the bus at the same time (bus fight)! So we use the 1802's INP instruction to do the write cycle. INP sets /MWR=0, but does not put data on the bus. Instead, the 1802 loads whatever is on the data bus into its D register.

Thus we have a way to read any byte in the upper 32k of the EPROM. The upper ROM address is specified by the 1802 R(X) register (with bit A15=0). This byte can then be used or saved in RAM normally.

The 9/20/23 update shows an example program to move blocks of data from 32-64k in the EPROM into RAM.