Teletype 43

Paul Koning paulkoning at
Wed May 24 07:26:03 CDT 2017

> On May 23, 2017, at 9:20 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.

Great to hear of the progress.

From what I remember and from what the Wikipedia article mentions, +- 10 volt is certainly fine and RS232-compliant.  I remember PCs that used TTL levels (0 and +5 or so) which clearly is not valid (not to mention that TTL level ICs are likely to fry when confronted with valid RS232 inputs such as -15 volts).

Your null modem experience makes sense.  There are a bunch of different signals whose meaning is often not clear, and older equipment is more likely to want all of them.  CTS is the main one, but while you might argue that DCD (carrier detect) shouldn't be required, it isn't too surprising that a terminal might look for it.


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