IAI America REXT - Manual (Page 144)


Part 1 Specification
Chapter 3 Gateway R unit
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Part 1 Specification
(2) Communication Method
The Modbus protocol uses the single-master/multiple-slave communication method. Only the master can issue a query
(to start communication). The slave receives a query, performs the specified function, and returns a response message.
The master can issue a query to a specific slave or broadcast a query to all slaves.
In the case of a broadcast query, the slaves only performs the specified function and do not return a response message.
Each slave returns a response message only after receiving a query addressed specifically to the slave. The query
transmission format consists of the slave address (or broadcast specification), function code defining the request, and
data/error check fields.
The response message transmission format consists of the request check field and data/error check fields. The
transmission formats of query and response messages are shown below.
Query and response communication cycle
(3) Serial Transfer Mode
The serial transfer mode in the Modbus gateway mode is the RTU (Remote Terminal Unit) mode.
In the RTU mode, 1-byte (8-bit) data is transmitted directly, which results in greater transmission efficiency.
Query message
(from the master)
Address
Function
Data
Error check
Address
Function
Data
Error check
8 bits
8 bits
N x 8 bits
16 bits
Response message
(from the slave)
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