Hugo Holden
Veteran Member
As most here know I have written a couple of articles on the pet now. One on the 9" VDU and how to restore it and another on how to locate faulty DRAM IC's. These articles refer to the PET that I have which is the one known as the Dynamic Pet, the NON-CRTC type that uses 2114 SRAMs and 4116 DRAMs.
I am not in a position to write up other PET variants because this is the only PET computer I have. And you really have to have the computer you are writing about on your desk to make sure any remarks are 100% accurate.
I turned my attention to the Character Address Generator. It is an interesting circuit (described by @daver2 as a horrendous piece of logic). One thing that makes it more difficult than usual is the circuit loops and very narrow pulses involved that are difficult to image on an Oscilloscope.
The obvious loop on the schematic involves the 8 bit latch and the counter IC's. But there are other loops. Also there are the interesting NEXT pulses. These are a form of a Reset pulse extracted from pulse streams and formed by flip flop control. These are much more elaborate (smarter) than the average reset pulse (This is the first time I have been able to incorporate a remark from Yogi Bear into one of my technical articles)
So that I didn't miss an extremely narrow pulses in the pulse streams which would have fouled up the analysis, I designed and built a Pulse Counting Logic Probe, which is described at the end of the article.
The idea of this article, like the others, is to help people repair PETs. In the case of the Character Address Generator, if you know what pulses are supposed to be there and why, it makes it much easier to look for them.
I am not in a position to write up other PET variants because this is the only PET computer I have. And you really have to have the computer you are writing about on your desk to make sure any remarks are 100% accurate.
I turned my attention to the Character Address Generator. It is an interesting circuit (described by @daver2 as a horrendous piece of logic). One thing that makes it more difficult than usual is the circuit loops and very narrow pulses involved that are difficult to image on an Oscilloscope.
The obvious loop on the schematic involves the 8 bit latch and the counter IC's. But there are other loops. Also there are the interesting NEXT pulses. These are a form of a Reset pulse extracted from pulse streams and formed by flip flop control. These are much more elaborate (smarter) than the average reset pulse (This is the first time I have been able to incorporate a remark from Yogi Bear into one of my technical articles)
So that I didn't miss an extremely narrow pulses in the pulse streams which would have fouled up the analysis, I designed and built a Pulse Counting Logic Probe, which is described at the end of the article.
The idea of this article, like the others, is to help people repair PETs. In the case of the Character Address Generator, if you know what pulses are supposed to be there and why, it makes it much easier to look for them.
