Siemens SIMATIC S7-1200 - Page 1211

Communication processor and Modbus TCP
13.5 Modbus communication
S7-1200 Programmable controller
System Manual, V4.2, 09/2016, A5E02486680-AK
1211
Extended_Addressing
The Extended_Addressing variable is accessed in a similar way as the HR_Start_Offset
reference discussed above except that the Extended_Addressing variable is a Boolean
value. The Boolean value must be written by an output coil and not a move box.
Modbus slave addressing can be configured to be either a single byte (which is the Modbus
standard) or double byte. Extended addressing is used to address more than 247 devices
within a single network. Selecting extended addressing allows you to address a maximum of
64000 addresses. A Modbus function 1 frame is shown below as an example.
Table 13- 87 Single-byte slave address (byte 0)
Function 1
Byte 0
Byte 1
Byte 2
Byte 3
Byte 4
Byte 5
Request
Slave addr.
F code
Start address
Length of coils
Valid Response Slave addr. F code Length Coil data
Error response
Slave addr.
0x81
E code
Table 13- 88 Double-byte slave address (byte 0 and byte 1)
Byte 0
Byte 1
Byte 2
Byte 3
Byte 4
Byte 5
Byte 6
Request Slave address F code Start address Length of coils
Valid Response
Slave address
F code
Length
Coil data
Error response
Slave address
0x81
E code