ASR33 reader

From: Tony Duell <>
Date: Tue Apr 15 18:00:01 2003

> >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?

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.

> 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.

Received on Tue Apr 15 2003 - 18:00:01 BST

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