Teletype 43

Doug Ingraham dpi at
Wed May 24 10:26:08 CDT 2017

On Tue, May 23, 2017 at 7:20 PM, Jim Brain via cctalk <cctalk at
> wrote:

> On 5/23/2017 1:07 AM, dave.g4ugm at wrote:
>> You probably need a null modem cable.
> I had one (and used it), but not all null modem cables are evidently the
> same :-)  I soldered up a loopback as you suggested, and the unit dropped
> into DATA MODE on startup.  I then played with the signals.  The unit
> really does want DCD to be active, as just doing CTS/RTS, DTR/DSR, and
> RX/TX did not do the trick.  Evidently, my null modem cable I had tested
> with previously does not connect DCD. Swapped out for a null modem adapter
> (from our old now deceased friend Radio Shack) and the unit works with a
> new Dell M4800 laptop (so "newer" style +-10V RS232 levels must be OK.

The minimum legit voltage swing for RS-232 is supposed to be plus and minus
5 volts and the
maximum allowed voltage swing is plus and minus 25 volts.  The problems
occur when gear didn't
bother to do the negative swing to minus 5 volts.  Plus and Minus 10 volts
is more than adequate.

The printer ribbon has less life left in it than I anticipated, but a list
> member is helping me, so it should be good to go after a deep cleaning and
> a light oiling.  I did notice the printhead starts to stutter at times on
> long lines, but I *think* it's a function of the damage to the ribbon, so
> we'll troubleshoot that only if it continues after ribbon fixes.

The ribbon on my 43 is a reinking type.  The ribbon loop is maybe 18
inches?  I have a new in bag
ribbon which I am sure is also dried out.  I used a drop of thin oil on the
reinking roller the last time
I messed with it and that seemed to work.  You probably want to use
something that will lubricate
because the pins on the dot matrix print heads do need that to keep them
from rusting and wearing

The "stutter" you mention is normal.  The printer electronics buffers a few
characters during the slow
carriage return and prints slightly faster than 30cps so when a new line is
started it goes at full speed
until the buffer is empty at which point it goes into the stutter mode.
This eliminated the need to send
nulls after a carriage return that was necessary on the earlier purely
mechanical printers.

Doug Ingraham
PDP-8 SN 1175

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