Just received a report of an issue with the Video board - in particular the address selection logic.
I have not personally experienced the fault myself, despite building a few version of the module -- but that doesn't mean much.
I have created a github issue with specific details to track the issue, copied below:
At least one person has reported an issue with V9958 Video board not working. They identified an issue with the address selection logic - in particular the diode based OR gates.
I chose to use diode base gates to reduce the chip count, but they can have issues with the fall time - which in this specific case, resulted in a board that would not work. (I have built a few iterations of the board, and never observed the issue myself, but really doesn't mean much!)
There were 2 fixes that each individually would resolve the fault:
- Change the 74HCT138 to 74ACT138 , or
- replace the 10K pull down resistor to a lower value - about 3.3K.
I have uploaded a pic to highlight the resistor that needed to be changed. PCB image
If be great to hear if anyone else has experienced this issue.