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What is a Null Modem?

Simply put, a null modem is an adapter or change that is made in an RS232 connection that swaps the transmit (Tx) and receive (Rx) lines.

With RS232 communication, each side of the connection has both a transmit pin (send), and a receive pin (listen). In order for communication to succeed, the transmit signal from Device A must connect to the receive pin of Device B, and vice-versa. The graphic below illustrates successful RS232 communication:

Tx from Device A connects to Rx on Device B, and vice-versa

The challenge with RS232 arises due to a lack of standardization among devices as to which pin is used for transmit (Tx) and receive (Rx). Essentially, every RS232 device has a 50% chance of having the Tx on pin 2 and the Rx on pin 3, or the Tx on pin 3 and the Rx on pin 2.

 Layout #1
 Layout #2

 Pin 2 – Transmit (Tx) 

 Pin 2 – Receive (Rx) 

 Pin 3 – Receive (Rx) 

 Pin 3 – Transmit (Tx) 

When the transmit signal from Device A is on the same line as as the transmit signal from Device B (both have the same layout from the table above), communication fails and nothing is received on either side of the link. The following graphic illustrates this failure:

Tx signal is on same line from Device A and Device B

In situations like the one above, a null modem inserted into the existing signal path will swap the signal from pin 2 with the signal from pin 3, resulting in successful communication. The following graphic illustrates the job of a null modem:

A null-modem inserted in the signal path swaps pins 2 and 3, resulting in successful communication
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  • 10-Apr-2019