15 Jul
2005
15 Jul
'05
4:50 p.m.
I'd posted here a while ago looking for info on a Tally 420PR tape
punch and have built a driver board to interface it to a PDP-8A.
But there is a problem with consistent punching/feeding (the
result is often a longitudinal tear instead of holes) even with
the pulse widths set to the recommended 4.5 ms +- 0.5 ms, -24
volts, and the feed mechanism intermittently binds up too.
Currently I'm running it with only the sprocket and feed drivers
hooked up, and simulated punch commands from a 555 timer, so the
result should be continuously feeding tape with just sprocket
holes punched in it.
From what I remember of this unit, the feed and punch
solenoids engage
clutches which then cause the apporpiate bits to move. If those
clutches
are held in engagement, the punch pins move up and down repeatedly, it's
not like a Facit 4070 where the solenoid moves the pin, and where the pin
would stay through the tape if the solenoid was kept engaged.
I think, therefore, that your problem must be mechanical. If the spocket
punch clutch engages late, the pin will be out of sync with the tape
motion. Maybe something is mechanically sticking. In any case I think you
need to strip things down, clean out the old lubricant, and check for this.
I suspect the problem is that the feed pulse starts immediately on
the falling edge of the punch pulse, so the pin hasn't cleared the
Why should it do this? Aren't you driving the 2 solenoids together at the
moment?
Is it possible that the back-emf spike from one of the solenoids is
causting thigs to mis-trigger? If so, you need to eliminate it. An
antiparallel diode on each solenoid will slow things down too much. Mybe
the old trick of diodes and a zener to clamp the pulse to a more
reasonable level would work (it's used in most tape punches).
tape as the feed mechanism starts to move. It also
tends to stick
in that position with the tape not moving (you can hear the
solenoids buzzing but the feed sprocket is not moving until
tweaked backwards a few degrees by hand). According to the
This certainly sounds like a sticking clutch.
schematic, the escapement solenoid (which allows the
tape to feed
one row per pulse) is supposed to be actuated internally by
contacts on the feed solenoid so there shouldn't be a timing issue
there.
Does anyone have more info on this punch model, or experience with
these asynchronous mechanisms in general? Is there a requirement
for a delay between punching the holes and pulsing the feed
solenoid? How long does it take for the pins to move up or down
From what I remmeber of the manual, you energise all
solenoids (punch and
feed) together. No delay.
-tony