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dinoboards / yellow-msx-series-for-rc2014

V9958 Video Board Designed for RC2014
MIT License
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V9958 Address selection logic may fail #1

Closed vipoo closed 1 month ago

vipoo commented 3 years ago

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:

  1. Change the 74HCT138 to 74ACT138 , or
  2. 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.

vipoo commented 3 years ago

I would not recommend doing this change, unless you specifically have this issue.