15 Apr
2003
15 Apr
'03
11 a.m.
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
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