Good Day,
I am trying to repair an Irrigation Sensor system that consists of a controller, Humidity/Temp Sensor, and a Daylight Sensor.
The system has been working fine for years communicating and powering through a 4 core .5mm Captyre cable, but now just started to show ERROR on the display
The 3 Modules communicate over RS485 using the SN75176BP transceivers.
There is a 120 ohm termination resistor in the Controller, and in the Daylight Sensor.
The Controller Polls the sensors, and then they respond with their data.
If I connect the 3 units together with short wires (less than 20CM each), it works fine. The controller shows me the Light Level, Humidity and Temp
But as soon as I extend the wire going to the Light Sensor (Using the Captyre Cable that they have always used, and have also tried Cat6 Ethernet Cable), then the controller no longer can read any of the sensors, it gives ERROR for the Temp/Humidity and ERROR for the light level.
Then what I tried is leaving the long cable in place, and using a 10cm jumper, to jumper across the +12v, and the GND, as well as the A and B Lines. As soon as I jumper the one specific Data Line, then it is happy, and the whole system works. If I instead jumper the other Data line, then it does not work.
So something is making it so that it will not work unless that specific line is short (but it is a differential signal, so I cannot figure out what would cause this).
Even if I disconnect the Temp/Humidity Sensor and just have the Controller connected through the 20m cable to the daylight sensor, it still does not work without jumpering that data line.
I have already replaced the caps on the modules, as well as swapping the SN75176 chips around, but no joy.
Has anybody come across something like this before, and where should I go from here?
Thanks
Peter