Modbus/TCP

This topic describes the RMC support of the Modbus/TCP Ethernet protocol. Whether you are using it to communicate with the RMC from a PLC or from a custom application, this information may be useful.

The Modicon Modbus/TCP protocol is one of the simplest protocols and has an open standard. This means the protocol specification is publicly available. See the Modicon Modbus/TCP web site at http://www.modbus.org for complete details on the protocol.

Supported Modbus function codes:

Read Multiple Registers (FC 3)
Read Input Registers (FC 4)
Write Single Register (FC 6)
Get Diagnostics (FC 8)
Write Multiple Registers (FC 16)
Read/Write Registers (FC 23)

Addressing and Data Format

See the Modbus Addressing topic.

Implementation Notes

While the Modicon Modbus/TCP web site has documentation and sample code for its Modbus/TCP protocol, it is worth pointing out a few specifics on communicating with the RMC over Modbus/TCP. This section neither replaces nor modifies the Modbus/TCP specification, but clarifies key points and defines the RMC's function support.

To create a connection with the RMC over Modbus/TCP, open a client socket with the RMC on TCP port 502. The RMC will only respond to Modbus/TCP requests on this port. Other ports are reserved or used by other protocols.
The Unit Identifier (Slave Address) field in the Modbus/TCP is only used by routing devices. Since the Slave Address is not used by the RMC, it should be set to zero (0) in requests to the RMC.

Typically, only functions 3 and 16 are used. Function 23 can be used to improve performance by doing a read and write in a single function. FC 4 and 8 were included for compatibility with existing Modbus/TCP masters, but otherwise add no value to the RMC.
The RMC handles incoming packets on a first-in first-out (FIFO) basis, making it possible to send multiple requests and then wait for the replies.
The RMC can handle up to 64 open TCP/IP connections at once. Typically each device uses one connection at a time, allowing the RMC to be connected to up to 64 devices at once.
Multiple-register reads and writes are limited to 100 16-bit words for writes and 125 16-bit words for reads—per the Modbus/TCP specification—even though the length field in the Modbus/TCP header makes it look like larger packets would be supported.
Floating point numbers are 32-bit values and use two (2) holding registers each. The least significant 16 bits comes first.