The Industrial Raspberry Pi Forum

Hello All,

I have problem to estabilish RS-485 communication on newly purchased RevPi Connect 5. I am trying to connect to the external Modbus RTU device from Node-RED insalled on RevPi.
Problem is followning:
When I connect directly RS-485 connector to my RS-485 (modbus rtu) slave, there is not possible to communicate with device (slave). BUT when I connect paralell to the RS-485 line another device (RS-485->USB), it starts to communicate between the originally connected devices. After long debugging I have discovered the problem is electrical. It seems the RevPi does not have the pull-up/pull-down resistors activated on the RS-485 data lines.
When no third device is connected, there is 0.8V between GROUND and D+ and the same voltage between GROUND and D-
If third device is connected, there is 2.3V between GROUND and D+ and 2.2V between GROUND and D-

The questions are:
- how to activate this pull-up/pull-down resitors, to RevPi will act as master?? Is there some command for it?
- how to activate the termination resistor? In the manual, there is only command for activation of CAN Interface termination resistror, but there is not command for enabling the RS-485 termination resistor.

+5V
|
+--[680Ω]--+--> A (D+) Pull UP resistor
| |
GND [120Ω] termination resistor
| |
+--[680Ω]--+--> B (D–) Pull DOWN resistor
|
GND


Thank you for your help!!!
Vlad.

Hello Vlad, well that's an interesting question, with RevPi Connect 4 there are switchable terminating resistors for the RS485 interface. This is how they work:
viewtopic.php?p=16547#p16547

Hi Dirk, Yes I have seen the video, which shows little bit uncomfortable way how to enable the erminating resistor.

But the main problem is the RS485 is not working at all. Please look to my first post - I am writing, the RS485 does not work if only RevPi and slave device is connected together. When I add third (no active) RS485 device, the bus starts working. My opinion is the pull-down and pull-up resistors is missing. But I cannot say if it is the reason. Ellectrically it looks like this.

Hello, thank you for your patience. Here I have a finished Python script from development for you: