Last semester I was given a decommissioned HP 1662As Logic Analyzer/Oscilloscope from MIT. This is quite a nice piece of test equipment, if not a little bit old (made in the early ’90s). 68 channels of 250MHz logic analyser and a 2 channel 250MHz oscilloscope are nice specs but a grey-scale CRT display and floppy drive show it’s age.

The trouble is that it is huge. At 442x218x330mm it is far to big to fly back to the UK with me. Considering the CRT takes up about 50% of the space inside I thought it would be an interesting project to try and replace the CRT display with an LCD monitor and shrink the whole package.

Pretty big

I managed to find the service manual.

From the service manual I learnt that the display had a resolution of 500×240 and that the CRT display was connected to the CPU board by only one connector, and so by disconnecting this cable I could remove the entire CRT assembly, hopefully replacing it with my own LCD electronics later.

Pin-out of the “sweep cable”

The signals that look key here are the VSYNC, HSYNC and Video signals which resemble those found on a VGA connector. This looks very promising as it seems the display data will be relatively simple to decode.

Now that I knew where to look, I wanted to see exactly what data was being transmitted.

With only a two channel oscilloscope, I could only look at two of the three signals at a time. First I looked at the HSYNC and Video signals.

HSYNC signal in red and Video signal in blue. I was using a 10:1 probe, so voltages are actually 10 times larger than shown.

This is pretty much what I expected. The HSYNC signal (red) is high most of the time however drops low at the end of every line, exactly every 40ns ie at a frequency of 25kHz. The Video signal (blue) varies while the HSYNC signal is high to describe each individual pixel. While VGA would normally have the Video signal being analogue between 0 (black) and 0.7 (full brightness), this video signal seems to only have a few values between 0 and 3.5V which makes sense as the display only shows 8 shades of grey. (Note that I am using 10:1 probes so the signals shown in the picture are 1/10th of the real value).

The VSYNC signal (10:1 probe)

Next I looked at the VSYNC signal which I would expect to drop low at the end of every frame. The signal does indeed drop low every 16.7ms indicating a screen refresh rate of 60Hz which is pretty standard.

With a VSYNC rate of 60Hz and an HSYNC rate of 25kHz, this implies that there are 417 lines per frame, however the datasheet suggested that there were only 240, so perhaps many of these lines are not actually shown.

More investigation is needed.