A project log for Stuart Conner's Simple TMS9995 board build

Stuart Conner's Simple TMS9995 board build

KeithKeith 10/03/2022 at 23:460 Comments

The DIP28 socket is wired for a 32K EPROM.

I prefer EEPROM because few people want to be bothered with UV erasers. I could mod the socket but would be limited to 32K. An alternative is to modify the socket so that pin 1 is switchable high or low, so that I can select either BASIC and FORTH using a 64K EPROM. Pin 1 is A15 on the chip, and VCC by default on the PCB, so I will put the standard BASIC in the second half of a 64K EPROM.

I have many 64K EPROM chips (type 27512). I don't plan to modify the firmware much, if at all, so I will not be bothered by UV erasure.

Links to ROM images:

EVMBUG system monitor and BASIC (32K)
EVMBUG system monitor and FORTH (11.5 K)

The latter not being 32K, I cannot simple catenate them into one 64K file, so I shall simply join them at the ROM-programming stage.


Programmed a 32K EEPROM AT28C256 with the monitor and BASIC.

Modified EPROM socket for 32K EEPROM


TMS9902 fitted, but no serial I/O seen.


EEPROM socket pin 27 changed from /WR to VCC. May have been corrupted, so I reprogrammed it. Got partly garbage characters with the common 9600-8-N-1 setting. Followed web page and set it to 9600-7-E-2 setting. Success, the sign-on message appeared!

Firmware is now working.