RS-485 modbus changing baud-rate not working

[tsotnep]

I set up communication between couple of those PLCs(Delta DVP-SS2), with baudrate of having value : 9600, in this case, it works well.

problem is that I'm not able to make it work on any different speed, I tested, lower and higher baudrates but nothing.

one thing to note is that I'm not using a resistor in network (I came across I would need 120ohm resistor, but have not tested it yet, its in the field, not so easy stuff). so whats your idea, what is the problem?

Ladder Logic: https://prnt.sc/vb7cfw
protocol description: https://prnt.sc/vb7ctu
config bits: https://prnt.sc/vb7efj

[kukuris]

Check list

1. Do u check that the DVP-SS2 <--> DVP-SS2 matching the baud rate / stop bit / parity bit / data bit ?
2. Do u check the communication error ?
3. Do u check with the communication simulation tool (communication data monitoring)
- simulate send/receive data with PLC

[tsotnep]

thanks for replying

1. yes definitely, i even disabled parity checking on all PLCs
2. As i observed, it was timeout error, i will test them in more detail on next attempt, i know all bits and registers for debugging.

3. no, i dont know how to do that, how do i sinulate tha th kind of thing?
4. what do you think about not having the 120 ohm resistor, can this be a problem even if its only 2 PLCs communicating on less than 1 meter distance?

[kukuris]

- Simulation tools
ex) Modbus Poll (Commercial but provide the trial version)
# PC(Modbus Master) <-- USB to RS485 Cable / Converter --> PLC (Modbus Slave)
ex) Self-made program (Based on PLC Modbus Protocol)

- Check the Modbus Master / Slave configuration (similar Slave ID) of PLC?

- Review the User manuals (PLC or ISPSoft)

- 120ohm (need for long distance prevent the signal distortion and attenuation), not essential for short distance