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LAB8/E changing characters


Veteran Member
Sep 30, 2013
Stockholm, Sweden
my name is Volker and I'm working in IT business since 22 years. Now that I'm in my fifty's, I found back to my roots and the computer I had when I was young. I realized it was fun and wanted to go further back in time. Since then I'm reading in this part of the forum, because I got very interested in DEC computers.
Last month I got a LAB8/E. I started to clean up everything in the LAB8/e, formed and tested the capacitors in the power supply. Calibrated the voltage, tested with some resistor load. I removed some minor problems with the backplane and started to put in the minimal configuration.
Spot on I was able to deposit and read, do some incrementing tests and so on. The serial card also worked and I was able to load the 4K Focal or Micro-8.
I got a second M8655 and a modern card with battery buffered memory. It did not took long and I was able to boot OS/8 for the first time. Thanks to all the people making such great tool possible!

That was very fast and straight forward, but suddenly the LAB halted.
Bit 4 of the address line was always lighted. After looking into the drawings I found a DEC8881 on the M8300 card that was driving this line and I was able to fix this.

But there is a further problem. While reading or sending from (all of the three) serial cards, the characters change.
While reading from the Terminal, it ends up to get something like 073x (x in 0-7) in AC.

Sending to the terminal shows like "xxxxx~~~~~~~~fffffffffffffffff`ffxxxxxxxxxxxxxx`xf~~~~~~~~~~~f~~\Q" regardless of what is in AC.

At the moment I have no idea how to find the bug. My configuration is: M8300, M8310, M8320, M8300, Shield, 4K Core, M8655 and the frontpanel. I borrowed some spare parts, but some of them are not OK. I changed the M8320, M8330 and the M8655 without changing the picture.
So I beg the problem is on M8300 or M8310.

Maybe someone of you can give me a hint?

Greetings from Berlin,

Hello Volker,

Do you have an oscilloscope? I would use that to have a look on the signal from the serial cards. To me it looks like there is a baudrate mismatch. Does it send this many characters when only sending one single character from the PDP-8? Do you know that the serial port adapter or terminal that you are using are working correctly?