13 Apr
2003
13 Apr
'03
8:39 a.m.
From: ard(a)p850ug1.demon.co.uk (Tony Duell)
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).
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 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
appears on the paper) and the next consecutive char is
then printed
correctly.
Presumably they must be entirely clear of the holes in the tape as they
come up through the tape.
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 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!