PDP-8/L teletype connection - test-drb@ccmp.vtda.org

21 May 2000

From: Lawrence LeMay &lt;lemay(a)cs.umn.edu&gt;
...
  The card is a W076 D. 

 I need to figure out which connections on the W076 are for what 'signal',
 so i can trace the wires to the DB-9 connector, and then make some
 adaptor cables. Does anyone have that information on the W076? My
 books dont list the W076. And which pairs on the W076 are active/passive,
 and am i correct in assuming that active pairs on one device connect to
 passive on the other end, ie, active supplies the current for the loop?
  It took me a while to remember where they stuck the schematic for the W076
but I found it.  If you get the 8/I maintenance manual from
http://highgate.comm.sfu.ca/pdp8  volume I Figure 5-7 pg 5-16 shows the 
schematic for the W076 and the connections to the teletype.
I see that the page was scribbled on, the numbers from top to bottom are
4 6 3 relay- 7 relay+.

...
  I would want to connect a Teletype model 33 (assuming
the one i'm 
 supposed to be getting one of these days actually works...). Also, 
 I would like to be able to connect an IBM to the 8/L, and hopefully
 use it to download paper tape images (probably using a rs232/current-loop
 interface). Has anyone been using an IBM like this in place of a 
 ASR terminal?
  If you get an ASR-33 which wasn't used for a PDP-8 I have instruction
on how to add the reader run relay.

I have used a PC (ok it was an old TI PC, used a different serial port
which supported hardware flow control which I connected to the reader
run signal) to talk to my 8/I, I built an external current loop 
converter.  If you need more information on this email me, from
later postings it looks like you might have a current loop card for
the PC.

If you get overrun problems it may be due to not having the reader
run signal.  The 8/I serial port is not double buffered so it doesn't
have much time (1-2 bit time) to remove the character before it is lost 
by the next.  If I remember correctly only some things like Focal
couldn't keep up at 110.

David Gesswein