Siemens SIMATIC S7 System Manual page 582

105BCommunication processor
12.5 Modbus communication
Table 12- 46 Write data functions: Write remote I/O and modify program data
Modbus function code
05
06
15
16
● Modbus function codes 08 and 11 provide slave device communication diagnostic
● Modbus function code 0 broadcasts a message to all slaves (with no slave response).
Table 12- 47 Modbus network station addresses
Station
RTU station
TCP station
Modbus memory addresses
The actual number of Modbus memory addresses available depends on the CPU model,
how much work memory exists, and how much CPU memory is used by other program data.
The table below gives the nominal value of the address range.
Table 12- 48 Modbus memory addresses
Station
RTU station
TCP station
Modbus RTU communication
Modbus RTU (Remote Terminal Unit) is a standard network communication protocol that
uses the RS232 or RS485 electrical connection for serial data transfer between Modbus
network devices. You can add PtP (Point to Point) network ports to a CPU with a RS232 or
RS485 CM or a RS485 CB.
Modbus RTU uses a master/slave network where all communications are initiated by a
single Master device and slaves can only respond to a master's request. The master sends a
request to one slave address and only that slave address responds to the command.
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information.
The broadcast function is not available for Modbus TCP, because communication is
connection based.
Standard station address
Extended station address
Station address
Standard memory address
Extended memory address
Standard memory address
Write slave (server) functions - standard addressing
Write one output bit: 1 bit per request
Write one holding register: 1 word per request
Write one or more output bits: 1 to 1968 bits per request
Write one or more holding registers: 1 to 123 words per request
Address
1 to 247
1 to 65535
IP address and port number
Address range
10K
64K
10K
S7-1200 Programmable controller
System Manual, 11/2011, A5E02486680-05