What is it?

An EPROM Adapter Board for the HP 3456A Digital Voltmeter

Why did you make it?

Over time, the mask ROMs in the early HP 3456A Digital Voltmeter's MC6800 Outguard CPU Board can sometimes drop bits causing erratic operation or failure. Both HP NOS mask ROMs and their more generic programmable versions can be hard to find and hacking modern replacements to fit seemed too heavy handed.

What makes it special?

• Minimally invasive. By installing sockets in the U5 ROM and the U6 Decoder locations, this EPROM Adapter Board can be installed with no etch cuts or wire jumpers allowing the use of a modern memory device in a wonderful old Voltmeter. To return the Voltmeter to original configuration, simply unplug the EPROM Adapter Board, install known good ROMS and reinstall the decoder chip.
• Selectable. If a 27C512 EPROM is used, two different versions of the firmware can be installed and selected using a shunt on the Adapter Board.

What is delivered (see first photo above)

• Board, as pictured in first photo, fully assembled.
• Two mating dip sockets to be installed in Voltmeter.
• Shunt for selecting image if a 27C512 with multiple images is installed.

What is not

• You supply the solder skills necessary to remove the existing mask roms and decoder chip from the voltmeters Outguard CPU Board, and install the supplied sockets.
• You supply the EPROM, image, and programmer. Images are freely available with a bit of help from Google.

Voltmeter Preparation (see second photo above)

• Remove mask ROMs U5, U7, and U8 from Outguard Board.
• Remove 74LS138 Decoder U6 from Outguard Board.
• Solder 24pin socket into U5 location.
• Solder 16pin socket into U6 location.

Adapter Installation (see third photo above)

• Source and program an EPROM with desired image.
• Install EPROM Adapter Board into sockets soldered into U5 and U6 locations.
• Install programmed EPROM into Adapter Board.
• Inspect work.

Operation

The shunt selects the lower image if a 27C512 device is used. Leave the shunt off if a 27C256 device is used or if the image in the upper half of the 27C512 device is desired.