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Since we are simulating the use of a bridge panel, we need to use TCP protocol for reliable data transmission.

Modbus operates according to the common client/server (master/slave) model. That is, the client (master) sends a request telegram (service request) to the server (slave), and the server replies with a response telegram. If the server cannot process a request, it will instead return a error function code (exception response) that is the original function code plus 80H (i.e. with its most significant bit set to 1). 

PLC1 is 158.38.140.56. It will be a master.

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https://help.codesys.com/api-content/2/codesys/3.5.13.0/en/_cds_f_networkvariables/
https://forge.codesys.com/forum/en/124/genmed-Network_Functionality_V23.pdf

https://www.prosoft-technology.com/kb/assets/intro_modbustcp.pdf

Source:/Helpful links:

Configuring a Network Variable Exchange:
https://help.codesys.com/api-content/2/codesys/3.5.13.0/en/_cds_configuring_network_variables_exchange/

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