Mike_Z
Veteran Member
Over the summer I picked up an M8650YA board. The past week or so I've been attempting to repair it. Last week I found that the delay line was bad. I tried to repair it, but it was beyond repair. I found that Roland Huisman designed an active replacement. I have ordered parts and boards and am currently waiting for them. In the mean time, I decided to study the operation of the M8650YA. I had a general idea of how it works, but thought that with a better understanding, trouble shooting may be easier. I have gotten through the addressing, the clock, the operation decoder and now I'm looking at the I/O buffers. The TTI and TTO both use 2 DEC 8271 chips. They are parallel/serial shift registers. I found a description in my 1973 maintenance manual. Yet there is no description of the R0 pin #1. I believe that it is a reset for the RC FF's? But what does it do? Set each bit to a 1 or a 0? Then I looked at my schematic and traced the R0's back. They are connected to a bunch of 7474 pin #13, which is an async reset. Could the 8271 R0 that also? Thanks Mike