Why one modbus device (PLC) has two address (Slave)

[ino_measure]

Hello, I'm new modbus. I try to study modbus application in LabVIEW by one example (Other person developed but now I can not contact to ask these question). The example code as bellow:

The device I want to communicate is PLC (TSXP57 103 Schneider) through PCMCIA 114 card and one converter from 485 to RS232.

I don't understand why in this code has two Acquision modbus.VI with two different Slave addresss (1 and 2 as above code). What is slave address I need to configure for my PLC (1 or 2)? Because I see only one PLC in this system.

The block diagram of Acquisition as bellow

As above blok diagram, I understand that LabVIEW will read input register from start address 500 with 45 register. This my understand is correct? If yes, PLC program need to store it's data at address %MW500 or %MW30500 (Because the register address of  "Read Input Registers" command from 30001-39999 as my understand)

Please tell me what is wrong in my understading!

Thank you

[Michael G · Accepted solution]

Greetings ino measure.

If I understand your application correctly, I believe you should be setting the slave address to 1.  If you would like to learn more about this modbus implementation in LabVIEW, I highly recommend visiting the link below as that article covers the use of this particular library.  Hope this helps!

Introduction to MODBUS

Regards,

Michael G

[ino_measure]

Hi Michael G,

Thank for your reply. My problem come from a misunderstand. The diagram code I posted was written for system with two PLCs. They modified hardware system but not yet finish modified software. After study and test on the hardware, I already solved the trouble. You are right, I should set up the address to 1 and delete one of them (Only one PLC on new system).

Thank for your help.