Version 1.1 external RGB+HS output issue.

[psmart]

Version 1.1 of the 40/80 column colour board was ready until I discovered an issue I hadn't seen on v1.0 of the board, primarily because I never used RGBHS but composite sync.

The issue is in the external Vertical Sync and Horizontal Sync separation, the separator circuit originates from Oh! Ishi’s most excellent museum site http://retropc.net/ohishi/museum/COL80A.PNG and separate’s as expected. Unfortunately, and it may just be my MZ-80A motherboard, there is a glitch on the Horizontal Sync which stops some digital monitors from recognising the signal. The attachment below you can see the issue, Yellow is Horizontal Sync, Blue is Vertical Sync, Purple is Horizontal Blanking and Green is Composite Sync. Just before Vertical Sync (Blue) goes active (low) the Horizontal Sync mimics the Composite and the Composite signal itself doesn’t look right, it comes from the mainboard, it shouldn’t have two pulses. The Horizontal Sync should be uniform, Ie a pulse every 64uS. Will figure it out, eventually, but it may need additional parts soldering onto the card to resolve. I looked at using a diode OR gate between the output of the multivibrator and the input to the XOR gate, increasing the timing on the LS123 multivibrator to ~44uS but that wasn't successful.

The double pulse occurs at the start of the Vertical Sync active period and again at the end.

Anyone using the circuit please be aware of this issue until I find a resolution (zero or minimum parts as I have already made up boards) and update the schematic.