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.
In 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.
- Pins 2 & 7 with shield(6) to Pins 2 & 7 with shield(6) to monitor data received by master device.
- Pins 8 & 3 with shield(4) to Pins 2 & 7 with shield(6) to monitor data transmitted by master device. Note 8=2, 3=7 and 4=6 on this side.
Here is a chart of what type of connection is on each of the four ports of the tap pictured.
| Port | Type | Use |
|---|---|---|
| A | Tributary | Controlled Device |
| B | Master | Controller |
| Y | Tributary | Monitor (A Tx / B Rx) |
| Z | Tributary | Monitor (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.
