Overview
Simply put, a null modem is an adapter or change that is made to an RS232 connection that swaps the transmit (Tx) and receive (Rx) pins.
Straight-Through
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 through a straight-through cable:
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.
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Pin 2 – Transmit (Tx) |
Pin 2 – Receive (Rx) |
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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:
Crossover
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 by creating a crossover cable (also called a null modem cable). The following graphic illustrates the job of a null modem:
Just Add Power Null Modem Jumper
Some models of Just Add Power have a built-in null modem jumper. The two positions swap the transmit and receive pins on the RS232 connector. The exact position is different for each model.
