What is a Null Modem Cable/Adapter?

Question:

• What is the difference between Null Modem and Straight Through?
• Straight Through vs Null Modem Cable/Adapter

Environment:

• Serial Communications RS-232 and RS-485
• Mobile Delivery

Answer:

What is a Null Modem cable/adapter and why is it needed?

A Null Modem cable or adapter is sometimes called a "crossover" cable. This nickname describes what is happening to "pairs" of wires inside the cable/adapter. These "pairs" of wires are just pairs of some sort of transmit and corresponding receive protocol. In serial communications there are two types of devices; a Primary "DCE" and a Secondary "DTE". The Secondary DTE device is designed to listen on the transmit lines and respond back through the Primary's corresponding receiving pin. There are many applications where you want to connect two DCE Primary devices together or DTE Secondary devices together. This is where you need some sort of Null Modem cable or adapter to "crossover" those transmit and corresponding receive protocols. For example some Bluetooth to Serial Adapters (Parani's) need a Null Modem connection to talk to the Meter

What is the difference between Null Modem and Straight Through?

I have wired up two db9 pin breakout boards in Null Modem and Straight Through to show what is happening inside the cable/adapter. Please note that the colors are not accurate and are only color coded for visual representation of the "corresponding pairs".

• Straight Through:
  Notice how all of the 9 wires are going straight across to their corresponding pin on the other side.
  
  
  
• Null Modem "Crossover":
  Notice how 3 pairs of wire crossover and the green (ground) wire is straight through. Also note that the 1 and 9 pins do not have any "yellow" wires. This is because they are not corresponding to each other and it is very common to not have either of these connections in a Null Modem adapter and they are typically not used.
  
  *FYI* It is common for DTE devices only need and have the red and green wires/pins.

Additional Info:

• Not all Null Modem cable/adapters are labeled Null Modem making it impossible to know if it is Null Modem or Straight Through.
• Do not have more than one Null Modem cable or adapter in a serial connection, two Nulls make a Straight.
• Please know that not all of the wires inside of a serial cable are being used when connecting devices together. It is even common for unused pins or wires to be missing from devices that do not use them.  For example, the Parani's Null Modem adapter is the only one that I have seen so far that actually passes the 9 pin through which can be used on the e:Count to power the Bluetooth adapter through the serial connection so you do not need a power cord (Please see Mid:Com or e:Count's documentation for more information and configuration in reference to that example).
• The entire serial connection does not need to be Null Modem. It is hard to find perfect one piece serial connections these days. So you can put a Null Modem adapter/cable in a series of cables, couplers,  db9 to db25, to make the whole serial connection a Null Modem one.
  For example: Here is a Null Modem db9 pin to db25 pin connection.
  
• General rule about serial connections from device A to device B is like plumbing, try to get from point A to point B with as few connections as possible to limit potential failures in the line.