Author: Roy Eassa
Date: 11:24:14 05/22/02
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On May 22, 2002 at 14:11:40, Rajen Gupta wrote: >On May 21, 2002 at 11:19:49, Vincent Diepeveen wrote: > >>On May 20, 2002 at 17:32:36, Roy Eassa wrote: >> >>with windows programs nullmodem cables work. >>with dos programs they do not work. >>difference between auto232 cable and nullmodem cable is >>that pin 1 & 2 are flipped >> >hi vincent: what is a nullmodem cable? what do i ask for when i go to a computer >parts store? is the word nullmodem cable a commonly used term? > >thanks Competent salespeople should know what a null-modem cable is. Normally, two major categories of equipment talk to each other via a serial cable (a normal "straight-through" cable): Data Terminal Equipment (DTE) such as computers, and Data Communication Equipment (DCE) such as modems. The pins a DTE uses to send data and signals are the same ones the DCE uses to receive them, and vice-versa. But when you want to connect 2 DTEs using a serial cable, a normal cable would not work because both computers would be trying to talk on the same pins. So you get a "cross-over" cable, in which the sending pins of one end are connected (crossed over) to the receiving pins on the other end, and vice-versa. This cable simulates having a pair of modems and a phone line between the two computers, so it has come to be called a null-modem cable. PC----Modem---//----Modem----PC In the above setup, each PC would connect to its corresponding modem using a normal or "straight-through" serial cable. PC--------PC In the second setup, you'd need a null-modem cable to simulate the missing data communication equipment (modems & phone line).
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