Cannot change the modbus settings on 3WL circuit breakers.

Dear fellow PCS7 users,

I came across your topic and decided to post another question in it as I am facing a similar problem.

I have a number of 3WL circuit breakers in an MCC cabinet connected to a Sentron PAC meter through Modbus over RS-485. I can see all the devices downstream from the PAC meter but the connection is unstable and breaks down often.

I am trying to decrease the baud rate of the PAC meter and the 3WL circuit breakers to see if that will fix the issue. However, I cannot change the modbus settings on he circuit breakers.

The part number of the 3WL Modbus communication module is:

                              3WL9111-1AT15-0AA0

Does anybody know how to set up this module?

Regards,

Datigo Turia

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Split from Modbus RTU communication with Sentron WL COM 16.

Hello, 

Datigo Turia

Dear fellow PCS7 users,

I came across your topic and decided to post another question in it as I am facing a similar problem.

I have a number of 3WL circuit breakers in an MCC cabinet connected to a Sentron PAC meter through Modbus over RS-485. I can see all the devices downstream from the PAC meter but the connection is unstable and breaks down often.

I am trying to decrease the baud rate of the PAC meter and the 3WL circuit breakers to see if that will fix the issue. However, I cannot change the modbus settings on he circuit breakers.

The part number of the 3WL Modbus communication module is:

                              3WL9111-1AT15-0AA0

Does anybody know how to set up this module?

Regards,

Datigo Turia

but the connection is unstable and breaks down often...

This is situation that isn't very clear from your description. Simply many other things can initiate this and is expected that you describe better what this unstable functionality means. For more cases it is not correct handling inside PLC code and also bad cables which can do faulty data transfers in some interference areas (electro/magnetic).

1.  Baud Rate

With regard to changing baud rate settings, I could not find a manual for the comm module, but in my experience whatever mode is used to change serial settings (software, DIP/rotary switches) the change is not recognized until power is cycled for the unit.   That might be the case or not, I don't know.  The approach is legitimate.

2.  Ground Loops

A huge problem with RS-485 communications is common mode ground loops created by different ground potentials between the multiple units on an RS-485 bus.

The signal reference wire, the 3rd wire in 2-wire RS-485 should be used for each device on the bus.  When connecting multiple devices, the 3rd reference wire can be at a different ground potential, which when connected them together creates a ground loop which can affect all the devices on the RS-485 bus.  (Ethernet avoids this because Ethernt communications is inherently, each isolated from the others by using a "magnetic" coupler.)

Siemens is well aware of this problem, spelling it out in the 2nd paragraph on page 170 of the circuit breaker manual, here:
https://cache.industry.siemens.com/dl/files/740/21465740/att_995408/v1/92399994174_201908201154493814.pdf

They recommend grounding the 3rd wire reference line at one end only, which in practice can be impossible because many Modbus RTU (RS-485) devices either do not provide a 3rd wire connection or their 3rd wire connection is grounded to the device itself.  Siemens infers that the 3rd on this comm module, but it connects to other devices on the bus, so the ground potential of the other devices comes into play.

In practice, you might find that the only means of stabilizing the RS-485 comms is to use RS-485 repeater/isolator modules in order to eliminate common mode ground loop problems.

3.  Wiring

Any RS-485 wiring in a noisy electrical environment needs to shielded twisted pair cable, with minimal length of "stub" wiring between the bus drop location and the device itself.

4. Biasing

I mention biasing because lack of biasing can cause periodic and intermittent faults on an RS-485 bus.

Profibus DP connectors incorporate what generic RS-485 call biasing, with a couple resistors that connect one of the driver lines to the +5V supply and the other driver line to 0V, in order to stabilize the idle state on the RS-485 bus.

The DB-9 connector for the RS-485 COM16 module does not provide the +5V or 0V connections to make biasing connections.  This is not unusual for not-engineered-from-the-ground-up-for-RS-485 comm devices.

The use of /repeater isolator modules usually fixes biasing issues.