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Functions codes
6.1
Modbus Commands
Modbus addressing model
The module allows R/W access to the following standard Modbus data register blocks:
• Coils (ref. 0x address range)
• Holding Registers (ref. 4x address range)
I.e. the module will not support the other standard data register blocks:
• "Discrete Input" (ref. 1x address range)
• "Input Registers"(ref. 3x address range)
Commands
Broadcast communication from master to slave(s) through device address 0 is supported. No
response from the slave is generated in that case. Broadcast communication is not secured by
the normal check mechanisms and shall be limited to very few uses.
A SITRANS F Modbus slave only recognizes (and accepts) few Modbus RTU commands/function
codes.
Supported function codes are listed in the table below.
Table 6-1
Function codes
Function code
01 hex
03 hex
05 hex
10 hex
11 hex
Modbus RTU RS 485
Operating Instructions, 03/2021, A5E00753974-AB
Command text
Read coils
Read multiple registers
Write single coil
Write multiple registers
Report Slave ID
Description
Reads the status of single bit(s) in a slave
Reads the binary content of multiple 16-bit registers in the slave. The
maximum number of registers is 26
Writes a single on/off bit
Preset values into a sequence of 16-bit registers. The maximum num‐
ber of registers is 25
The SITRANS F slave will respond to a Report Slave ID command
(Command 17) request from the master by giving in formation about
device type, vendor, revision level etc. in a format as shown
6
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