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How You Switch Asynchronous Signals with a Null Modem

To achieve acceptable asynchronous signal conditions, you may need to install a device known as a null modem. Most asynchronous devices communicate using a wiring protocol called RS-232. One side of a connection is called the data terminal and the other side is called the data set. As long as your host device is wired as a data set and the COM ports are wired as data terminals, there should be no communication problems. However, if the host you are connecting to is wired as a data terminal, mismatching signals will result. Consequently, you need to insert a null modem. Use the following components to establish this type of connection:

• 9-pin or 25-pin null modem (a null modem can also be purchased with a built-in gender changer)
• Standard cable
• 25-pin to 9-pin converter (you may not need this)
• Gender changer (if the connector of the standard cable is incorrect)

Note: When RS-232 signals are mixed up, one-way communication results.