top of page
A Drop-In Replacement for Tektronix 7854 ROMs (MCM68766)

A Drop-In Replacement for Tektronix 7854 ROMs (MCM68766)

The MCM68766 is used in lots of vintage equipment, including the Tektronix 7854 oscilloscope. Suffers from ROM rot, and it is hard to get a replacement. Why struggle with the UV-erasable version when you can have a drop-in replacement made from a contemporary EEPROM? The board presented here has about the same size as the DIL socket, and can be programmed using standard programmers built for the MCM68766, as the board has provision for handling the high programming voltages involved. If you want direct access to the onboard EEPROM, three of the EEPROM pins (!OE, !WE and !CE) are brought out to a header provision so you can use those instead of a high-voltage programmer.
I have firmware for the 7854 and will program these for you. If you have firmware to program the boards with, I will gladly program them at no additional charge.
Speed: Note that the fastest variant, the MCM68766-C30, has a maximum access time of 300ns. The EEPROM featured on this board, the Atmel AT28C64B-15TU, has a max access time of 150ns!
Check out the video showing this at work.


Note that the "hit until set" algorithm traditionally used for EPROMs may not work with an EEPROM. A single-pass algorithm is what is required. Make sure your EPROM programmer supports this.