The PET Video Adaptor

This document describes a small circuit that allows you to connect a CGA, EGA or digital multisync monitor to your CBM or PET computer's Userport (2000, 3000, 4000, 8000 series). Since the circuit does not convert scan rates, your monitor must be able to handle the frequencies the computer produces. Especially with the 8000 series this may require a true multisync monitor. This circuit buffers and optionally inverts each ouput signal, so many monitors can be connected without any negative effects towards the internal monitor of the computer.

Building the hardware

I created a single sided PCB layout for the circuit, so it should be relatively easy for everybody to make an own PCB. All the connections on the upper side of the board can be made with wire links. For my prototype I decided to make another  small piece of single sided PCB that has just the solder pads for the Userport plug. This is then glued to the top of the main board and makes it thick enough to allow both sides of the Userport plug to be soldered in place. The layout files were created with Cadsoft's Freeware version of Eagle 4.09 and are supplied together with Postscript and TIFF files for convenient printing without having to download Eagle yourself.

The following components are needed:
IC1 74LS86 (quad xor gate for inverting the signals)
IC2 74LS245 (octal bus transceiver to buffer the signals)
S1 triple DIP switch  
CN1 Userport plug  
CN2 female DB9 socket  
CN3 dual pin header +5V supply (e.g. from Datasette port)

Here are two pictures that show the device and the part diagram:
The prototype, still missing CN3 Part layout (click to get larger version)
final product

The final product (only 10 made)

Using the Adaptor

Just plug the adaptor onto the Userport of your PET, connect a +5V source to CN3 and connect your monitor to CN2. EGA and Multisync monitors must be set to 8 or 16 color mode, or the resulting picture may have a blue background. If the picture is not good, invert each signal with the DIP switches until the picture is good (1: video signal, 2: vsync, 3:hsync). Some monitors use the sync polarity to switch between (color) modes, others need the correct polarity on both sync signals to lock onto them at all.

I use this adaptor mostly for repairing old PET and CBM computers, because the internal monitor can then be removed from the unit to allow easier access to the mainboard. Sometimes it's also unknown if the monitor still works at all, and this adaptor offers some help in testing this. I also prefer not to power the monitor up and down very often when running tests on the mainboard itself.


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