Siemens SIMATIC S7-1200 System Manual page 1167

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Table 13- 53 Write data functions: Write remote I/O and modify program data

Modbus function code

● Modbus function codes 08 and 11 provide slave device communication diagnostic

information.

● Modbus function code 0 broadcasts a message to all slaves (with no slave response).

The broadcast function is not available for Modbus TCP, because communication is

connection based.

Table 13- 54 Modbus network station addresses

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 13- 55 Modbus memory addresses

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.

S7-1200 Programmable controller

System Manual, V4.2, 09/2016, A5E02486680-AK

Communication processor and Modbus TCP

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

Standard station address

Extended station address

Station address

Standard memory address

Extended memory address

Standard memory address

13.5 Modbus communication

IP address and port number

Address range