>From: Vintage Computer Festival <vcf_at_siconic.com>
>>From Section 5.4: Signal Characteristics:
>
>"The standard interface is supplied as the output of TTL type 7404.
>Signal sense is specified as a Logical TRUE and is a positive level;
>logical FALSE is a ground level. Circuit characteristics are shown in
>figure 5-2."
>
>This excerpt can be found on Page 46 at:
>
>http://www.pdp8.net/pdp8cgi/query_docs/tifftopdf.pl/pdp8docs/74226-300dpi.pdf
>
>(warning: this server is a bit slow)
>
Seems fine from here :-). It's a mighty 486-133 (and meets the 10 year rule)
with a 384k DSL. Most of the documents have a link to a copy mirrired on
highgate which has better connectivity. I assume it was the speed of
downloading the documents you were commenting on.
>Figure 5-2 is approximately this:
>
> o Output Drive
> | TTL type 7416
> /
> \ I Logical True
> R / load Iload = -400 ua _at_ 2.4V min (with R=5.6K)
> \
> |
> +--------------->
> |
> /
> | I Logical False
>-----| sink Isink = 16 ma _at_ 0.4V max
> |
> \v
> |
> +---------------<
> |
> GND
>
>So I was told before that the way this works is that a signal will be
>output as "Logical True" voltage when the data is a '1' and a "Logical
>False" when the data is a '0'. But the way the connector is wired is
>confusing.
>
>Each of the 12 data lines (for reading the holes on the card) has 2 pins:
>the "data" pin and then the "data return" pin. See page 45 of the URL
>above.
>
The return is the ground reference.
>I imagine the data pin goes into one of my inputs on the 6522 (which
>handles TTL levels). However, I was told by Jim Willing that I should use
>an optoisolator in between the output of the reader and the 6522 because
>the output voltage is clamped mechanically(?)
>
It appears to be driven by a TTL Logic. Page 10-3 shows the output connector
which data lines go to the control card pg 10-20 and the returns go to the
DC return (power supply ground). From page 5-6 which you got the output
diagram from it says that twisted pair cable was recommended with the
returns terminated close as possible to the data receivers. Running
the grounds with the data lines in the twisted pair like that gave a little
more noise immunity. The control card parts list (PG 12-31) seems to have a
couple versions, normal TTL (7404), open collector outputs (7416) with pullups
or open collector with inverted data (7417).
>But where does the RET
>(return) pin go? To ground?
>
Ground it to the ground of your receiver card.
>Do I really need the optoisolators? Is the
>6522 robust enough to just take the raw signal from the reader output?
>
I wouldn't bother with the optoisolators and I don't think the PDP-8 card
did either. Anybody have a spare M843 CR8E card reader control
or schematics for it? Polling from an Apple ][ should be slow enough that
it won't see the glitches that are likely to happen on transisions of the
index mark signal. I frequently will read strobe signals like that until
I get the same value twice then act on it.
David Gesswein
http://www.pdp8.net/ -- Run an old computer with blinkenlights.
Have any PDP-8 stuff you're willing to part with?
Received on Thu Dec 26 2002 - 14:16:00 GMT