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What did I do to my PDP-8 today.

Now that the PC04 is working I can load FOCAL and the 4USER Patch and run it with 2 Teletype ASR33. It's fun to do so. But the main Teletype did not load any short FOCAL program.
I noticed, the Reader Run relais is closed all the time. So it loads much too fast.
I opend the relais and inserted a new reed glass into the coil. So this is now working again.
Then I also noticed, the READER is not reliable enought. About one or two Bytes of 100 Bytes are wrong. So I began adjusting.... Long story, no real progress.

There are so many adjustment points:
Trip magnet 2x
Reader Magnet 1x
Reader 4x (or even more)

Is there a recipe how to find the correct adjustment for an ASR-33?
 
gnupublic said:
Is there a recipe how to find the correct adjustment for an ASR-33?
Click to expand...
I maintain 3 33's. They all act up in unique ways. I go with figure out what the exact symptom is such as is a single bit error, multiple etc and then try to figure out what could be causing it. Last problem reading tapes was really the contact block in the keyboard broke so was moving around and contacts were occasionally closing while tape was reading.

gnupublic said:
Now that the PC04 is working I can load FOCAL and the 4USER Patch and run it with 2 Teletype ASR33. It's fun to do so. But the main Teletype did not load any short FOCAL program.
Click to expand...
Early FOCAL's have a design problem. They run the teletype off interrupt and don't drop reader run when the small buffer fills they just generate the overflow error code. Since each character is echoed if the teletype sends data faster than the serial echos back to the teletype you will get the overrun after a while loading a program. With the earlier 8's where baud is controlled by I pot I just adjusted it to make 8 send faster than Teletype. I assume your using later machine with crystal clock so don't have that option. Can't remember if baud rate I adjusted was slow or teletype was faster than exact 110 baud when I adjusted.
 
djg said:
Early FOCAL's have a design problem. They run the teletype off interrupt and don't drop reader run when the small buffer fills they just generate the overflow error code.
Click to expand...
Turns out that's kind of all you can do on the early machines. If you try to return after a KRS, the flag is still set and the interrupt is re-triggered. If you KRB, then reader run is set. There isn't a KIE, to allow you to mask the interrupt. So input continues until the buffer overflows.

(I actually do know a way to work around the problem, but if would take some explaining, and there's no version of FOCAL that does it, to my knowledge.)
 
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djg said:
Last problem reading tapes was really the contact block in the keyboard broke so was moving around and contacts were occasionally closing while tape was reading.
Click to expand...
Oh, this is very unexpected.
djg said:
Can't remember if baud rate I adjusted was slow or teletype was faster than exact 110 baud when I adjusted.
Click to expand...
Maybe it's worth to think about an own "Reader Run generator" inside the ASR-33 to slow down the reader to a reasonable frequence.

For testing the READER with FOCAL I made a endless papertape with "@". So when I read this in there ist mostly one M instead of an @ in a line. I will do this with other patterns next weekend.
 
I repaired a M8300. There were two DEC 8266 and one 74151 broken. Repairing these modules helps understanding the 8.
 
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