Modbus connection testing through Modscan

[charzree]

Dear All,


I could not get the results with modscan software to test the Modbus connection for the digital relays. The modscan software says “Modbus message timed out”. I have connected the LAN Cable (CAT 6, Untwisted pair) to the RS-485 terminals (Ground, A-, B+).


Here is what I have done:
Physical connection:
1. Lan cable (with RJ 45 connector and all the 8 cores connected inside the RJ 45 connector) connected to TSX CUSB 485.
2. The other end of LAN cable connected to the relay RS-485 terminals (ground, A-, B+).
Here, Core No: 8 connected to Ground
Core No: 4 connected to B+
Core No: 5 connected to A-
3.USB connection of TSX CUSB 485 connected to PC
4. The rotary switch of TSX CUSB 485 is set at position 3 (other direct)
Software setting:
All the parameters are matched for the modscan software with the digital relays (Baud rate:9600, Parity: Even, Data bits and stop bits settings not available to set in the digital relay but set as 8 and 1 respectively in modscan software.


Where am I wrong?

 

[danw]

If memory serves me right, "Modbus Message time out" indicates a "no connection" error, like the wires are not connected or connected backwards.

A common problem with 2 wire RS-485 is that the line label designations are different, vendor to vendor. Some vendors call (+) A, others call it B.

In a perfect world, one connects A to A, and B to B. In the real world sometimes one connects A to B and B to A.

If backwards, it will not damage the drivers, but it won't work.

Swap the lines and see if you get a hit.

There's a lot of other issues that can happen though.

Are you addressing the correct slave ID (Modscan calls it "Device ID")?

My suggestion is to make the length 4, and use an address one less than the desired register address, use Hex data format (Define Data) and look for which registers correspond to the data you should be seeing.

Do not use the any leading number (4) for the register addressing, for instance for holding register 400138, just use the index value 138. The leading numeral (4) is not part of the Modbus message, it's used for humans to identify the register as a 'holding register', and Modscan has you select the register type separately (Modbus Point Type).

Do not use the screen that displays the traffic. That's too confusing, unless you're troubleshooting message formatting.

You want the screen the displays just the register number and the value returned.

 

[Ken Roach]

For those of you watching at home, the TSX CUSB-485 is a USB to RS-485 converter that was originally made by Schneider Electric as an interface to older HMI and PLC devices, using the Uni-Telway protocol.

It might also work as a general-purpose RS485 interface; I have no direct experience with it but have read the User Manual: [Link]

There's a function-selection rotary switch that should be set for Position 3, allowing it to be used for Modbus.

On the 8-pin connector, it has only Data 0 (B) on Pin 4, and Data 1 (A) on Pin 5.

Pin 8 is the data common terminal. Many users omit this, but it needs to be part of any "2-wire RS485" connection. If the digital relays have no dedicated signal common wire, connect this pin to the system ground at the digital relay.

I agree that the A, B, +, and - labels are inconsistent on RS485 devices from different vendors. If the digital relays have only two wires, it can't hurt to switch them and see if the link works correctly.

 

[danw]

Thanks for the link, Ken.

I assume the 560 ohm "polarization" is what everyone else calls "bias".

Charzree: Are you sure your digital relays are Modbus RTU (8 bit data word) and not Mosbus ASCII (7 bit data word)