For ease of construction and testing, I built this project on a solderless breadboard. For permanent installation, I'd use other construction methods.
From the Xerox 820 documentation, we can learn the following details of the video signal, which is on a "2x5" standard header marked J7:
- Pin 3: vertical sync
- Pin 4: horizontal sync
- Pin 5: video data
- Pin 6-10: GND
Make the following connections:
- J7-3 to Feather D11
- J7-4 to Feather D10
- J7-5 to Feather D9
- J7-10 to Feather GND
A more permanent solution could use a permaproto board with the IDC Breakout Helper 2x5 and an IDC cable. Having all the wires would be helpful, because it would keep a GND signal between each of the high speed signals, for better signal integrity.
In the photo, I have used bare jumper wires inserted into 1-row and 2-row connector housings to make the connections nicer, but this is not strictly necessary.
Now, plug the Feather into a compatible monitor, and apply power to the Feather and the Xerox. After a moment, the Xerox's boot screen should be displayed.
Note that it's normal for the text to be shifted towards the top right hand side of the screen.
Tuning the pixel sampling to remove jitter
Often, jitter by 1 or 2 pixels will be seen, affecting the latter part of a line more than the start of the line.
This occurs because the clock inside the RP2040 is not exactly in sync with the pixels from the Xerox 820.
If the amount of jitter is bothering you, you can try the following things:
- Try adjusting the constant
fine_pixel
in the range 0 to 9. - Try adjusting the constant
pixel_frequency
, starting with a change of ±100000 (the granularity of this adjustment is about ±40000)
Because each Xerox 820 and each Feather RP2040 have slightly different clock rates which even vary with temperature enough to affect results. Some experimentation will be required to get the most stable display. Commercial video converters have sophisticated circuitry in order to automatically fine tune the conversion from analog to digital, while in this case it's necessary to make do with blunt tools like changing numbers manually until getting the best result possible.
Another possibility is to pick up the pixel clock directly from U14 pin 8 (modifying the PIO program to wait for this edge before sampling each pixel). I did not investigate this method, as I got adequate results without modifying the Xerox PCB.
Note: Powering the Feather
It is not recommended to supply the Feather with 5V power on the pins, so it will need to be powered by a USB cable.
The best option for internal power is to cut open a USB "C" cable, find the 5V wire, and connect it to a source of +5V power on the Xerox 820 PCB, such as the "+" leg of capacitor C314, a silver capacitor near the main power connector. The Feather is not designed to accept 5V input on the "USB" pin, only via the USB connector, and is not protected against back-powering or having two power input sources on the same pin.
Page last edited March 29, 2024
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