ASR33 reader

From: Ethan Dicks <>
Date: Wed Apr 16 12:29:01 2003

--- Tony Duell <> wrote:
> No, that's the standard Teletype automatic reader control relay. The
> relay I am refing to is a DEC modification

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

> > 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
> 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.

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")

       JMP LOOK

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.


References (all available through

   "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).
The New Yahoo! Search - Faster. Easier. Bingo
Received on Wed Apr 16 2003 - 12:29:01 BST

This archive was generated by hypermail 2.3.0 : Fri Oct 10 2014 - 23:35:44 BST