geoffm3
Veteran Member
My PCjr is having some trouble and I'm trying to figure it out. The system beeps twice, and doesn't display anything. After looking through the BIOS listing, I discovered the POST test can be setup to spit out some error codes to the built-in serial port (at 9600 8-O-2). So, I set this up tonight to try that out, and sure enough, it spits out three bytes:
<FF><04><FF>
If I read the BIOS listing correctly, this is memory failure in the built-in 64k RAM. I poked around on the RAM with my oscilloscope and everything looks about like you'd expect... except Dout from each RAM chip. I see that they go to the logic 1 state just fine, but when it looks like it should be low, it looks like something else is driving onto the bus... the state is somewhere between 0 and 1. Looking at the schematic, I can't figure out how this could happen, as there are several latches that use this bus (labeled ED{0-7} on the schematic), but it doesn't look like anything else should be able to output onto that bus.
Has anyone encountered this before?
<FF><04><FF>
If I read the BIOS listing correctly, this is memory failure in the built-in 64k RAM. I poked around on the RAM with my oscilloscope and everything looks about like you'd expect... except Dout from each RAM chip. I see that they go to the logic 1 state just fine, but when it looks like it should be low, it looks like something else is driving onto the bus... the state is somewhere between 0 and 1. Looking at the schematic, I can't figure out how this could happen, as there are several latches that use this bus (labeled ED{0-7} on the schematic), but it doesn't look like anything else should be able to output onto that bus.
Has anyone encountered this before?