PDP-11/84 SLU problem?

Johnny Billquist bqt at update.uu.se
Tue Dec 16 02:18:31 CST 2014

On 2014-12-15 18:06, John Rollins wrote:
> So in function checking the PDP-11 now that the Series/1 is out of the basement and I have room to move the unit around and plug it in, I have discovered that it doesn't appear to transmit data from the console port on the back. I have tried two terminals(WY99) and two different PC's(Windows laptop with hardware serial port and a Mac with a USB adapter) with various software, a half dozen different cables, and every conceivable switch setting on the PDP with nothing to show for it aside from a lot of time spent the last couple of weekends.
> So when I power up, the LED display on the front counts down through the numbers and stops at 4. As near as I can tell, this is something of a good sign. But nothing ever comes up on a connected terminal. Sometimes pressing keys on the terminal results in the RUN light turning off, other times it does nothing. This would seem to indicate that the PDP knows that something is trying to talk to it, but it is not sending data out.
> Short of spending an entire day disassembling half the machine and tracing every wire hoping to find something out of place, does anyone have any ideas on what may be happening here, or what to look for?

Well. The first to check is if you have expected levels on the pins of 
the serial port. At idle it should hover at about -12V, although the 
RS-232 spec allows for significant variation. But it should be an 
obvious negative value anyway.

Also check that your terminal also asserts such a value. And that if the 
terminal does it on pin 2, then the PDP-11 should do it on pin 3. 

What I am thinking is that the level converters might be blown. Not 
uncommon in older systems. If so, that is all you need to replace. It's 
normally a single chip.

This also makes sure you are not using broken or wrong cables. (Oh yeah, 
and ground is on pin 7.)

If this don't help, then we need to dig further...


Johnny Billquist                  || "I'm on a bus
                                   ||  on a psychedelic trip
email: bqt at softjar.se             ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol

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