ASR33 Teletype interfacing

From: John Lawson <>
Date: Fri Oct 3 20:51:19 2003

 HWat you call 'rattling' sounds suspiciously like "running open" - the
selector magnets in the TTY have no current applied across them, hence the
machine is always 'spacing', or rather, all the internal parts are
rotating, rather than waiting quietly for the next pulse.

  I don't see in the Daedalus site exactly how the 20 (or 60) millamps of
current is going to be supplied from a regular RS232 port...

  Your 33 has a "local loop supply" giving current to the selector magnets
when you have the function knob in LOCAL position - then, as you say, it
becomes a heavy, noisy, accessorized typewriter.

  When you switch the knob to LINE, it is expecting to be supplied with
"loop supply current" from it's communication line. This current, normally
'on', is broken by the pulses representing the data, and thus the selector
magnets operate the mechanism to print each character as it comes from the

  You really oughta should get a copy of the ASR33 manuals - they're
pretty common, but first you need to somehow supply loop current to your
TTY... this requires the right kind of interface - especially if you hook
it up to a PC to test, otherwise, you're liable to drive quite a high
voltage back into the PC and kill it dead - while the ASR33 just sits
there and laughs.

  Or 'rattles', as the case may be...

  Fear not! You are quite close to getting it working... and if it types,
punches, and reads back tape sucessfully, then the machine sounds like
it's in pretty good shape and ready to work.


Received on Fri Oct 03 2003 - 20:51:19 BST

This archive was generated by hypermail 2.3.0 : Fri Oct 10 2014 - 23:36:22 BST