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Troubleshooting Serial Communications

422 Swap adapter for SMPTE 207m

Pin swap to reverse Master and Tributary control lines

In the broadcast industry the 422 serial interface is commonly used to communicate between equipment. The 422 wiring standard comes in two types, Master or Tributary. These types designate which pins in the standard db 9 physical interface have the signals present and by adhering to these standards, manufacturers of different equipment have a way to communicate by using a standard 9 pin cable that is wired pin to pin. Sometimes it is required to convert from one physical interface to the other and an adapter is a good thing to have in your tool box.

422 Swap adapter

Below is a diagram to reverse a Master device to a Tributary device. In summary, pin 2 goes to pin 8 and pin 7 goes to pin 3 to move the receive signals while pin 3 goes to pin 7 and pin 8 goes to pin 2 to move the transmit signals. The shield associated with receive is pin 6 which will go to pin 4 while the shield assoicated with the shield associated with transmit is pin 4 which will go to pin 6. Pins 1 and 9 are frame ground and pin 5 is a spare that is not defined. Note, this only addesses the physical interface, if you try to connect two devices that are not supposed to talk, the physical layer is not the only thing that needs to be changed.

graphic showing pin connections
In this example, the grounding / shielding and spare pins are shown as optional connects, please use these pins as required for your installation. Note that at least one of Ground or Shield must be common between the two systems.

The SMPTE 207m standard covers communications over the RS-422 db9 interface and you should read the SMPTE document in detail if you want to know more about how the communications are intended to work.