The PET 6540/6550 Adaptors

These two simple adaptors replace the 6540 ROM and the 6550 RAMs in the display circuit of original PET 2001 computers only. These adaptors will not work to replace system RAM or ROM chips in the same machine, this is what the 6502 RAM/ROM expansion is for. The 6540 adaptor takes any EPROM from 2716 to 27C512 without any need for jumper changes (but in larger EPROMs the data must be at the correct address), and the 6550 adaptor replaces the two display RAMs with a single 2016, 6164 or 61256 SRAM chip (also narrow versions). There is a single jumper to configure the board for different RAM chips.

Building the hardware

The PCBs are double sided and require some skills to be built. The layout files were created with Cadsoft's Freeware version of Eagle 4.09 and can be downloaded here. The file contains layouts and schematics for other projects, too. You can also buy the PCBs from me, just visit the order page.

The following components are needed for the 6550 RAM adaptor:
IC2 74LS00 SMD (combine R/W with Phi2)
IC1 SRAM 6116, 6164 or 61256
precision socket adaptor 22 pin
precision socket adaptor 22 pin

These components are needed for the 6540 ROM adaptor:
IC1 EPROM 2716-27C512
precision socket adaptor
28 pin
4.7kOhm resistors 0805 package

The 6540 adaptor can optionally be equipped with some switches and pull-up resistors to allow the selection of different character ROM images. You can use this to hold both the BASIC1 and BASIC2 versions of the character set and maybe sets of national characters. Up to eight different images can be selected, depending on the size of the EPROM you use. The jumper wire connectors are mapped as follows:

EPROM type
2732 and above
2764 and above
27128 and above
J4 ground connection

Note that you have to cut the trace between the solder pads of the resistor and install the resistor if you want to make use of the extra character ROM banks, otherwise you will produce a short circuit if you close the switch between J1-3 and J4! If you need only 2 or 4 banks, but have a larger EPROM, just leave the resistor pad(s) corresponding to the extra address line(s) alone. Address lines A14 and A15 as present on 27256 and 27512 EPROMs are always connected to logical high. Remember this when burning the ROM images into the EPROM.

Here are some pictures of the PCB layout and the adaptors themselves:

The 6540 adaptor The dual 6550 adaptor final adaptors in action


If you have questions about this project,  you can mail me: please visit my contact page.