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Getting know Modbus TCP mapping to remote I/O on Vision OPLC


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Good day everyone!

We're a SI (that has not much experince  with the Unitronics PLCs.. )  and  met Vision PLC HMI panel 1040 in the field.

We need to read some I/O points from the remote I/O modules, connected to the OPLC's i/o exp. port .  For integration with another system. 

From the FAQs I've read the module allows 4 connections, and Modbus TCP 502 port as well.  Please advise if there are some mapping  rules for remote i/o modules to the OPLC's Modbus TCP memory? 

Is this mapping is set in the project (that we don't have...) or there is some formula like,  X (pos_of_the_module)* mem_space_occupied_by_the_module + offset inside the module, to get the MB TCP address of the remote i/o point? 

Thanks for help in advance.

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  • MVP 2021

The digital I/O on the expansion modules is numbered sequentially starting at I-32 for inputs and O-32 for Outputs.

All the information you need to know is in the help file. Search on MODBUS Slave Addresses. Click on MODBUS,IP, then Slave Addressing. There you'll find the addresses for the different operands in the PLC, including the addresses for the I/O.

You'll need access to the PLC program unless the program is already set up to be a MODBUS Slave.

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Hi, thanks for your reply.

Could you please lead to the help file or manual where this info is...  I think it is configured as a slave (server) on the TCP port (which I'm going to use), becuase there is a client somewhere in the network that works with it.  But the point is the mapping of remote i/o points to mb_tcp points  - is automatic ?  (i32 --> (matches)  MB Digital Input 0, I33 --> MB DI1...., O32 --> MB_Coil 0 etc...) OR  matching has always to be done in the project configuration.

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  • MVP 2021
7 hours ago, Flex727 said:

Search on MODBUS Slave Addresses. Click on MODBUS,IP, then Slave Addressing. There you'll find the addresses for the different operands in the PLC, including the addresses for the I/O.

See above on where to find the info in the Help file. While you could certainly map the I/O directly to MBs in the program, it is not automatic. The Help file provides the addressing for the MODBUS Master to find the I/O coils. They would be addressed just like any other coils, just higher up in the address table.

Being a MODBUS Slave is not sufficient to be able to read data from the PLC. You must have a SCAN_EX function block running in the program to scan for incoming MODBUS requests.

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