ASR33 reader woes

From: Tony Duell <ard_at_p850ug1.demon.co.uk>
Date: Sun Apr 13 15:39:00 2003

> From: ard_at_p850ug1.demon.co.uk (Tony Duell)
>
> >May I ask a very simple question. What machine are you using this with
> (from the description below, I assume a PDP8). If it _is_ a PDP8, do you
> have the reader control relay mod fitted correctly? If not, then you
> _will_ drop characters I think.
>
> My computer is an 8/L. I do not have the relay fitted at all. After a
> careful examination of the schematic and manuals, this relay only forces TTY
> power on, in LINE mode (this is redundant if the TTY is already on and

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).

> >So what you're saying is that it sends the correct number of chracters,
> but some of them are corrupted to 0's?
>
> Yes, when an error occurs while printing the test tape I can "hear" the
> nonprinting character being sent to the typing unit (it "jumps" but nothing

Do you know if it's corrupted to 0's, or if odd bits are mangled?

> appears on the paper) and the next consecutive char is then printed
> correctly.
>
> >Either your read pins are misaligned (they should come through the middle
> of the tape holes --
> check this), or the contacts are sometimes not making properly.
>
> Agreed. They look centered to me (but what IS the definition of "centered"
> i.e. at what point in the read cycle? The tape and pins do not seem

Presumably they must be entirely clear of the holes in the tape as they
come up through the tape.

> perfectly stationary with respect to one another during the read cycle.)
> The manual shows that the sprocket wheel should be rotated forward to take
> up the slack against the detent at which point the pins are centered in the

Could it bee too much free play here? In other words the sproket is not
being held firmly during the read cycle? Check the detent adjustment??

> Who's Heath Robbinson? I always liked Rube Goldberg's inventions...

Heath Robbinson was an English artist who produced drawings of such
inventions. Over here the term 'Heath Robbinson
Idea/Invention/Device/...' is used to describe anything that somewhat
crazily designed....

>
> >3) focus a camera on the reader mechanism. Set the aperture appropriately
> for the flashgun. Make the room dark, hold the shutter open on B, let it run
> 4) When the flash fires, close the shutter, turn everything off
> 5) Develop the film and hope that it shows you what the problem is.
> -tony
>
> Sounds awfully complicated. Perhaps I can do this with my Sony digital
> camera if it has a sync input!

This depends on the length of delay from the sync input to actually
taking the picture (this may not be that short!). The advantage of using
a flashgun in a darkroom is that the trigger time _is_ short. That's
quite apart from the fact that I am alergic to digital cameras...

-tony
Received on Sun Apr 13 2003 - 15:39:00 BST