--- Tony Duell <ard_at_p850ug1.demon.co.uk> 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")
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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Received on Wed Apr 16 2003 - 12:29:01 BST