Functions codes
6
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 Command text Description
01 hex Read coils Reads the status of single bit(s) in a slave
03 hex Read multiple registers Reads the binary content of multiple 16-bit registers in the slave. The
maximum number of registers is 26
05 hex Write single coil Writes a single on/o bit
10 hex Write multiple registers Preset values into a sequence of 16-bit registers. The maximum num‐
ber of registers is 25
11 hex Report Slave ID 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
Modbus RTU RS 485
Operating Instructions, 03/2021, A5E00753974-AB 23
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