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Troubleshooting Serial Communications

422 tap

Occasionally it is neccessary to log the data on a 422 serial communications interface.

When this is needed, an adapter to safely pull the transmitted data off and send it to a dedicated receiver is advised.

422 TAPIn this type of adapter, only pins 2 & 7 are sent to the monitoring device, pin 6 is the shield for that data pair. The monitoring device is a special unterminated dual channel 422 receiver, typically with a LTC receiver to lock the time reference to the house time of day in use by all the other devices.

Visual representation of pins 2,7 and 6 to monitor data received by master device and pins 8=2, 3=7 and 4=6 to monitor data sent by master device.

Here is a chart of what type of connection is on each of the four ports of the tap pictured.

PortTypeUse
ATributaryControlled Device
BMasterController
YTributaryMonitor (A Tx / B Rx)
ZTributaryMonitor (B Tx / A Rx)

Since the 422 tap is bidirectional, it does not matter if the controlled device is on tap A or tap B (A=B, straight through) the data on port Y or Z will always be present. If ports A & B are swapped, then the data expected on ports Y & Z would swap also.

The SMPTE 207m standard covers communications over the RS-422 de9 interface and you should read the SMPTE document in detail if you want to know more about how the communications are intended to work.