Subject: Re: ASR33 reader

From: Charles <[email-masked]>
Date: Tue Apr 15 17:18:01 2003

On Tue, 15 Apr 2003 12:00:01 -0500, Tony Duell wrote:

>Ah. With the PDP8/e and PDP11s, there's a relay wired in series with the 
>reader trip magnet (the one on top of the distributor unit at the rear 
>right). It allows the computer to start and stop the reader, and in fact 
>the read a character at a time. It's partly controlled in hardware, in 
>that when the PDP derects an incoming character, it turns off the relay 
>(stopping the reader). It's then turned on a again by software 
>(presumably after said software ahs read the character).

My TTY *does* have a relay on the reader "powerpack" board which
can be turned on if an XON (DC3, Ctrl-Q) is sent to the typing
unit, and off again upon receipt of an XOFF (DC1, Ctrl-S); it also
latches itself on if the reader's manual start lever is flipped,
until it is pushed to the stop position, or the tape-out contact
opens. Is that the relay to which you are referring?

Are you sure the 8/e actually goes to the trouble of turning
off/on the reader after EVERY character? At 10 cps that's 100,000
uS between characters (60,000 machine cycles on my 8/L)?

Meanwhile I have my 33 apart awaiting new function levers that
trigger the above flow-control options (someone had installed
Ctrl-B and CR instead of the correct Ctrl-S and -Q). Further
reader "tuning" will thus be delayed...

-Charles

---

From: [email-masked] (Tony Duell)
Date: Tue Apr 15 18:00:01 2003

> opens. Is that the relay to which you are referring?

No, that's the standard Teletype automatic reader control relay. The 
relay I am refing to is a DEC modification (Intel had an almost identical 
mod that was used with the MCS8i (and probably other machines). It's a 
little PCB with a relay on it that's fitted to the call control unit. The 
cotnacts are wired in series with the reader trip magnet coil. The relay 
coil is wired back to the PDP (no connections to the teletype), and is 
driven by a transsitor circuit there.

> off/on the reader after EVERY character? At 10 cps that's 100,000

Yes. It's done partially in hardware. The relay (and hence the trip 
magnet) is turned off as a character is received by the PDP. The computer 
then has to set a bit on the serial card (I can remember how to do this 
on a PDP11, but not on the PDP8) to restart the reader.

-tony

---

From: Ethan Dicks <[email-masked]>
Date: Wed Apr 16 12:29:01 2003

--- Tony Duell <[email-masked]> wrote:
 
> relay I am refing to is a DEC modification 

Yes.  I've seen it referred to as the "LT-33" mod.

> on a PDP11, but not on the PDP8) to restart the reader.

In the case of the PDP-8, whether it's an M8650 OMNIBUS card, or the
M-series M706 or even the DTL W-series W706, it all works similarly.
The KCC IOT (6032) clears the Reader Run flip-flop that drives
the buffer that activates the relay.  For the TTL and DTL cards, there
are 3 combinable IOTs related to console input (the M8650 doesn't allow
combining the KSF with the other IOTs, but in practice, that wasn't done):

  KSF (6031) - Skip on Keyboard Flag
  KCC (6032) - Clear Keyboard Flag
  KRS (6034) - Read Keyboard Buffer Static
  KRB (6036) - Read Keyboard Buffer Dynamic (combined KCC + KRS)    

When the code running on the PDP-8 tells the console hardware to read a
byte from the console, the normal sequence of IOTs toggles the bit without
any extra programming required.

Example (from the "1966 PDP-8 Users Handbook")

LOOK,  KSF          /SKIP WHEN TTI IS FLUSHED
       JMP LOOK
       KRB          /READ TTI INTO AC

You can use a KRS to just peek at the TTI (TTY In) buffer register on
the console interface, but, since you would still have to clear the
buffer with a second instruction, it was rarely used.

AFAIK, the relay does not impinge on the keyboard path, just the stepper
relay of the papertape reader, so there is no such throttling of the
TTY keyboard.  For "modern" uses, people have made minor modifications
to the M8650 board to handshake bytes for, say, PeeCee-based papertape
emulators, but normally, it's hit-or-miss on overflow if you disregard
the relay output (eg. EIA/RS-232 cables from an unmodified M8650).

Non-console TTYs (DZ-11 w/20mA distribution panel, etc.) used a 4-wire
interface (2 current loops) and were not typically used to load data
via papertape.

-ethan

References (all available through www.pdp8.net)

   "dec-8i-hr2a-d-pr.pdf", page 35, or page "II-31" of paper
    document "D-BS-8I-0-11", titled "Teletype Receivers".

   "PDP-8/e/f/m Minicomputer Handbook", page 13 of the PDF scan of
   chapters 1-3, or page 1-13 of the paper version.

   "1966 PDP-8 Users Handbook", pages 72-73 of the PDF or pages 58
   and 59 of the paper copy).
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