PET A/V-Interface
Content
Introduction
The PETs have the video on the user port. This is the video signals and vsync and hsync. For the old PETs without a CRTC, it is actually an NTSC signal that can be mixed from it.
The new PETs (with CRTC) do not follow any video standard. They require a patch of the Edit ROM to be able to output a standard video signal. The disadvantage is, that with the altered settings, the internal monitor does not work anymore.
The source of this project is a schematic from the newsletter issue 1&2 of the Commodore PET Users Club of England.
Introduction
The PETs have the video on the user port. This is the video signals and vsync and hsync. For the old PETs without a CRTC, it is actually an NTSC signal that can be mixed from it.
The new PETs (with CRTC) do not follow any video standard. They require a patch of the Edit ROM to be able to output a standard video signal. The disadvantage is, that with the altered settings, the internal monitor does not work anymore.
The source of this project is a schematic from the newsletter issue 1&2 of the Commodore PET Users Club of England.
The circuit requires 5V supply voltage, which is not provided by the PET User Port. The circuit, that I have made, offers two options for this:
1. via a ribbon cable from a cassette port dongle.
2. a 5.5mm/2.5mm barrel connector and an external power supply
Option #1 might be the easier option, but some might prefer option #2.
Further on, the CB2 pin of the user port is often used as an output for sound. So, it makes sense to also add a voltage divider and two audio jacks to the circuit.
Since it might be good to access the User Port with other hardware (like a joystick), the circuit provides a feed through user port.
The CRTC usually outputs an inverted video signal, so there is a switch between an inverted and non-inverted video signal.
Circuit of the PET A/V-Interface.
The tantalum cap for C2 is required. An ordinary aluminium electrolytic capacitor will not work properly.
There are two ways of supplying 5VDC to the A/V Interface (since the PET user port does not provide any supply volktage.
- Via the cassette port and a cassette port adapter (same like in the C64/C128 and VIC-20 diagnostic harness)
- Via the barrel connector and a 5VDC power supply.
PET Audio/Video Interface Rev. 2 with Power Supply via Cassette Port Adapter
PET A/V Interface with Power Supply via the barrel connector
The AV-Interface working with the CBM3016 (no CRCT), a framemeister video-to-HDMI converter and a TV as display
Notes
The PET A/V interface works with an unmodified PET without CRTC (cathode ray controller), since this outputs a format close to an NTSC video signal. The 80 column (CRCT) PETs, like the CBM8032 produce a signal, which is not close to a standard vide signal. They require a modified Editor ROM with parameters for either NTSC or PAL video signals.
Steve Gray has collected and generated EDIT ROM parameters, you can find those here:
- The PET/CBM Editor ROM Project
- The CRTC parameters on github: cbm-edit-rom/crtc-reg-normal.asm
3D Printed Case
For Rev. 2, there is a 3D printed case with label stickers. The case is required in two versions, one for the ribbon cable (cassette port) supply and one for the barrel connector power supply.
The case for the barrel jack version is different from the cassette port powered versions
From left to right: PET Video Mixer Rev. 0 and Rev. 1, PET A/V-Interface Rev.2
Revision 0 was trying out the circuit of the PET Users Club of England. I noticed, that with the CRCT, at least the video signal was inverted, so in Rev. 1 the video signal and both sync signals were invertible with jumpers. That was accomplished with the 74LS86 XOR gates. In Rev. 2, the audio output was added (hence the new name PET A/V-Interface).
Github
The PET A/V-Interface will be published as open source hardware on github, as soon as the documentation is finished.