Modbus RTU Communication between CJ1W - SCU31 - V

[jpl 0]

hi all, First of all Merry Christmas and Happy New Year. I having problem to read the data from a 3rd party slave devices, a Digital Power Meter, via Modbus RTU, RS485, 2 wires system. i using Omron CJ2M-CPU31 and Omron CJ1W - SCU 31 - V, Ver1.3 I connected the wires 3rd party Omron SCU-31 Port 1 + ---- Pin 8 - ----- Pin 6 the Power digital is just sending the data, so from my site i just need to read the data, no need send any command. which protocol should i use in the program? Protocol Macros or Serial Gateway how should i set configuration of the communication board? will the LED of the card RD1 lit when communication established? Thanks

[DanW 41]

I'm a Modbus guy, not an Omron guy, but I can provide some insight into Modbus basics 1) The concept of a device 'sending' data is not a Modbus concept. Modbus is master/slave. Slaves are silent unless spoken to. Typically, a master requests specific data from specific slave register locations and the slave replies to the request with the specific data. The function code the master uses determines whether whether it is a read or a write operation. A Master can write data to a slave, but it is highly unlikely that a meter is a Modbus master. Meters are typically Modbus slaves that reply to a Master's request for data. The PLC is the Modbus master, which requests the data. You need to make sure the PLC can act as a Modbus master, unless you find that the meter is, in fact, a Modbus master. 2) RS-485 has no standard for which line is (-) and which line is (+). Some manufacturers label one way, others the reverse. If the lines are connected backwards, the line drivers are not damaged, but communications doesn't work. Once you're convinced that you're sending a request to the correct slave node address at the right baud rate and parity, but not getting a response, it pays to swap the A/B or (-)/(+) lines and try it again. The serial communication parameters of both devices must be identical - word size (8 bits for RTU), parity and stop bit. Dan

[Sleepy Wombat 27]

DanW is right. the meter will more then likely be a modbus slave.... that been the case you can use protocol macro... in the download section for omron there is a modbus protocol.. also there is a sample program called modscan which allows you computer to communicate modbus to a slave...(but remebmer tthat is rs232 so need a convertor)

[BobB 58]

The Omron SCU port uses pin 1 as - and pin 2 as +. Protocol Macro is excellent - download it and give it a go - do not forget to set up the serial port properly with CX-P.

[PMCR 66]

I think I can fill in the gaps. I am an Omron guy first, and a Modbus guy second. Jay Anthony taught me everything I know about Modbus (and most of what I know about Omron). I developed the generic Modbus RTU master code at the link below for someone who is not a 'Modbus Guy (or Gal)', but is willing to work with Modbus. Download the code. There is a manual that shows you how to setup the Omron PLC and fill in the DM areas to make an SCUxx function as a Modbus Master. It has 10,000 + downloads, so I believe that others are finiding it useful. http://forums.mrplc.com/index.php?app=downloads&showfile=867

[pfort 20]

I am one of thoses who downloaded and want to thank you for it.

[PMCR 66]

Great. Thank you for the feedback. As a user of the code (and documentation), can you point out anything that was not clear for a user implementation?

[pfort 20]

As i can recall, everything was clear and worked well, except it was slower than excpected. I dont have finish with this project because i had to work on something else for last 2 months. My project for this is with a CJ2M, and 3 3G3MX2 drives. Maybe when i will continue working on this i will have comment and/or question for you.

[Jay Anthony 118]

That's my boy...

[rob5029 0]

I'm Attempting to use this to talk to a Chiller Control panel, but the pannel needs to know the node address of the "supervisor" which would be the CS1W-SCU31-V1 module. I've never used Modbus before, and I've not found anywhere that specifies the node number for this module. Any idea where this might be/what it would be set to by default? Many thanks Rob.

[PMCR 66]

Rob That is interesting. On Modbus RTU, the Modbus RTU master does not get assigned a Node Number. Only the slaves do. Can you post the documentation that you are looking at?

[pfort 20]

I wanted to ask but forgot until now, i have set the baud rate to 57600 bps instead of 19200, it is faster and i dont seem to have any problem, should i expect some in the future?

[rob5029 0]

I've just had the Chiller engineer in, and I miss understood the Manual. It sed to set the Supervisor node number, which i assumed Supervisor meant Modbus Master, when infact the Supervisor is the unit that controlls all 3 chillers. The Supervisor will acts as both a master & slave so I only nead to read from the supervisor node and not all 3 chillers. havent tried it yet as there is a cable missing! hopefully that will be put in tomorrow Cheers Rob.