Siemens SIMATIC S7-1200 - Page 1283

Communication processor and Modbus TCP
13.9 Legacy Modbus RTU communication (CM/CB 1241 only)
S7-1200 Programmable controller
System Manual, V4.2, 09/2016, A5E02486680-AK
1283
For example, after MB_SLAVE is placed in a LAD network, you can go to a previous network
and assign the HR_Start_Offset value. The value must be assigned prior to execution of
MB_SLAVE.
Entering a Modbus slave variable using the
default DB name:
1.
Set the cursor in the parameter field and
type an m character.
2.
Select "MB_SLAVE_DB" from the drop-list.
3.
Set the cursor at the right side of the DB
name (after the quote character) and enter
a period character.
4.
Select "MB_SLAVE_DB.HR_Start_Offset"
from the drop list.
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- 160 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- 161 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