A dumb question

Can anyone tell me what a null-modem adapter is?

It is an adapter for a 9-pin RS-232 cable that reverses pins 2 and 3 (xmit/rcv) so that xmit on one end is rcv on the other end and vice-versa.

http://www.nullmodem.com/NullModem.htm I love Google!

Warning! Longer, more complex reply. RS232 was designed as the communication standard from Data Terminal Equipment (DTE - a 'terminal') to Data Communication Equipment (DCE - a 'modem'). The modem provides conversion to signals acceptable on the telephone line. At the other end would be another modem, connected via (presumably) another connection to a computer.

But people wanted to use the connection locally from computer directly to another computer. They could not be connected straight through pin-to-pin as the transmit on one computer would be directly connected to the transmit on the other computer. Other signals likewise would be connected to their identical counterparts and not to an appropriate pin. For example, handshaking signals RTS (Ready To Send) and CTS (Clear To Send) would either need to be cross connected or, in many cases, the RTS is simply locally connected to the CTS of the same unit.

Thus a 'null modem' cable provides the same effective connections as if a complete Computer-Modem-Telephone Line-Modem-Computer hookup existed. The minimal 'null modem' does indeed just cross Transmit and Recieve on one end to the appropriate opposite pin on the other. The 'modem control and handshaking' lines are dealt with in a variety of ways resulting in a number of designs for 'null modem' cables. One may not be right for all setups.

bernie_carlton said:

Warning! Longer, more complex reply. ...

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And I thought you would start whit the inventor of the telegraph!

Actually a great source for my information on serial communications is the book "C Programmer's Guide to Serial Communications" by Joe Campbell. And indeed he does start with the telegraph in his incredibly thorough discussions.

I'll have to think on this one a bit.

I LOVE telegraphy.

I HATE C.

73 de kc

CaseyK said:

I'll have to think on this one a bit.

I LOVE telegraphy.

I HATE C.

73 de kc

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I agree. Serial stuff is so much fun Anybody remember 300bps?

just commenting on the title of the original post...the only dumb questions are the ones that don't get asked!

plchacker said:

I agree. Serial stuff is so much fun Anybody remember 300bps?

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300bps was the new fast version. I think the original was 110.

How about the handset coupler with the two rubber circles that never seemed to fit right. And listening paitently to the tones waiting to catch a hiccup in the transmission.

I never go tot use the rubber cups, but I do remember them. 300 was the slowest that I used. I kinda Like serial. I have used it for so many different applicitaions that its not funny.

I found an old Hayes 1200bps Modem in my lab. Still in the box.


I agree, the only dumb question is the one that goes unasked.

300 bps is still used.
We do work for a major utility in the UK and some of their telemetry systems for remote site monitoring use the old telex lines and 300bps modems (SS300). Plenty fast enough for remote monitoring and alarms.

You had 300 bps?

All we had was 110 acoustic couplers

dit dit dit dah

keithkyll said:

300bps was the new fast version. I think the original was 110.

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Correction, 75 bps was the first version. It is still the first option, when selecting communicaton speed for a COM port on Windows XP.