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.
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:
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:
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.
