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Auxiliary Area and CIO Area Allocations (Modbus-RTU Slave Mode) Section 9-3
is turned ON. Counting will stop if they reach FFFF hex.
9-3-3 Supported Modbus-RTU Commands
The Modbus-RTU slave mode of the Serial Communications Board/Unit sup-
ports the following Modbus-RTU commands.
The frame format used in Modbus-RTU slave mode is as follows:
Note The CRC code is given in the order low byte, high byte.
Note 1. Setting the slave address to 0 indicates a broadcast command. Responses
are not returned for broadcast commands.
2. DM Area settings are read when the power is turned ON to the CPU Unit,
when the Serial Communications Board/Unit is restarted, or when the port
is restarted.
3. Program retry processing on the host computer (e.g., personal computer)
when a transmission error or other communications error occurs.
9-3-4 Command and Response Details
Read Coils
Function Reads multiple bits from the CIO, Work, Holding, or Auxiliary Area of I/O
memory.
Command
Note The address depends on the area that is allocated.
Function code
(hexadecimal)
Function Modbus name
01 Reads multiple bits from the CIO, Work,
Holding, or Auxiliary Area of I/O memory.
Read Coils
02 Reads multiple bits from the CIO Area of
I/O memory.
Read Discrete Inputs
03 Reads multiple words from the DM or EM
Area of I/O memory.
Read Holding Registers
04 Reads multiple words from the CIO,
Work, Holding, or Auxiliary Area of I/O
memory.
Read Input Registers
05 Writes a bit in I/O memory. Write Single Coil
06 Writes a word in the DM or EM Area of I/
O memory.
Write Single Register
08 Executes an echoback test. Diagnostic
0F Writes multiple bits in I/O memory. Write Multiple Coils
10 Writes multiple words in the DM or EM
Area of I/O memory.
Write Multiple Registers
Slave
address
Function code Data CRC
1 byte 1 byte 0 to 252 bytes 2 bytes (See note.)
Length Data
Function Code 1 byte 01 hex
Starting Address 2 bytes 0000 to FFFF hex (See note.)
Quantity of Coils 2 bytes 1 to 3E8 hex (1 to 1000)
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