Chibi Consulting
Overview of Serial Communications

Overview of Serial Communications in Broadcast TV equipment

RS-422(EIA-422) and RS-485(EIA-485) are commonly used in the broadcast industry to communicate between equipment using the SMPTE 207m (ST 207:1997) standard. The 422 protocol is used for point to point communications and the 485 protocol is used for multi-drop communications. These balanced, twisted pair serial connections typically terminate on d-sub 9 pin (DE9 Female) style connectors. Wikipedia (9-pin) article.

The 422 communications protocol utilize balanced pairs of connections for transmitted data and received data which means that common mode interference is reduced; as a result, 422 or 485 data paths can go quite a significant distance without problems. Even so, wiring issues can cause a significant amount of problems when setting up equipment. Most equipment that follows the SMPTE 207m (ST 207:1997) standard will use d-sub 9 male to male cables wired straight through or pin to pin. If you want to make your own cables, keep in mind that the correct pairing is pins 2 & 7 with pin 6 as the shield for that pair and pins 8 & 3 with pin 4 as the shield for the other pair. Frame ground is on pin 1 & 9 with pin 5 designated as a spare or not connected. {Pin 5 is not usually wired.}

If you are interfacing to any devices with other than SMPTE pinout; make the adaption to SMPTE pinout at that device and run correct shielded wires back to your equipment so that you are always using the correct twisted shielded pairing and common voltage reference.

A useful side effect of using a balanced signal is that the wiring status of a link can be checked without actually needing to decode the data. The 422 Tester page goes in to detail on how to make a tester for properly wired serial connections.

The links below contain details for special cases which you may come across when setting up or troubleshooting communications with broadcast equipment.

Master Tributary reverser or 422 swap adapter

A 422 swap adapter is used to connect two devices wired as Master or two devices wired as Tributary together.

422 tap adapter

422 taps are used to monitor communications while safely troubleshooting problems.

Computer adapters

This adapters document link is so you can find various 422/485 adapters which are available if you need to interface to a broadcast device serially. Let me know if you find others that should be linked. contact 24 at chibiconsulting dot com


The SMPTE 207m (ST 207:1997) standard covers communications over the RS-422 d-sub 9 interface and will be used as the wiring standard in this web site. You should read the SMPTE document in detail if you want to know more details about how the serial communications are intended to work.