FAQ: Termination and RS485
Q: Why is RS485 termination important?
A: Incorrect termination can cause a data communications line to stop working because it stops the receiver from filtering out background noise and can corrupt the signal. The RS485 line should be terminated with 120Ω at both ends, therefore if the control equipment end is terminated then the last dome on a line should also be terminated. It is ok to have several domes connected in a daisy chain configuration as long as the last one is terminated.If the control end is not terminated then two domes or two daisy chains can be connected to a single port.
Q: I have no control of the dome, what can I try ?
A: Check the obvious things first. Its surprising how often simple things are overlooked. Have you selected the correct camera and monitor using the control equipment ? Check you have power to the Dome LED's on the main board indicated power. Are the RS485 connections the right way around ?, check A & B lines. Check the address of the Dome and the telemetry protocol is set correctly in the address cradle.
Q: How can I check the RS485 data line ?
A: All Mercer Domes have three LED's located on the main PCB - (Apart from the LITE version which only has one) These LED's indicate the health and status of the telemetry data being send and received by the dome.There are three LED's : ACTIVITY this should flash on boot up and is OFF when the dome is idle. during normal PTZ operation this will be ON constantly.
COMMS - if OFF no data is being received. If flashing slowly un recognised data is being sent, if flashing more quickly data is being received but not meant for this address. ON means valid telemetry is being sent and correctly understood.
HEARTBEAT - Constant flash means a healthy communications network.