Mb_Slave - Siemens SIMATIC S7 System Manual

STATUS value
(W#16#....)
8188
8189
818A
818B
818C
8200
6.3.2.3

MB_SLAVE

The MB_SLAVE instruction allows your program to communicate as a Modbus slave using a
port on the Point-to-Point (PtP) CM 1241 RS485 or CM 1241 RS232 module. A Modbus
RTU master can issue a request and then your program responds via MB_SLAVE execution.
You must assign a unique Instance Data Block when you place the MB_SLAVE instruction in
your program. This MB_SLAVE Instance Data Block name is used when you specify the
MB_DB parameter on the MB_COMM_LOAD instruction.
Modbus communication function codes (1, 2, 4, 5, and 15) can read and write bits and words
directly in the PLC Input Process Image and Output process Image. The following table
shows the mapping of modbus addresses to the process image in the CPU.
MB_SLAVE Modbus functions
Codes Function
01 Read bits
02 Read bits
04 Read words Input
05 Write bit
15 Write bits
Modbus communication function codes ( 3, 6, 16) use a separate and unique Modbus
holding register data block that you must create, before you can specify the MB_HOLD_REG
parameter on the MB_SLAVE instruction. The following table shows the mapping of Modbus
holding register to the MB_HOLD_REG DB address in the PLC.
MB_SLAVE Modbus functions
Codes Function
03 Read words
06 Write word
16 Write words
The following table shows the supported Modbus diagnostic functions.
S7-1200 Programmable controller
System Manual, 11/2009, A5E02486680-02
Description
Invalid Mode value or write mode to read only slave address area
Invalid Data Address value
Invalid Data Length value
Invalid pointer to the local data source/destination: Size not correct

Pointer to a type safe DB type DATA_PTR (must be a Classic DB type)
Port is busy processing a transmit request
Data area Address range
Output 1
Input 10001
30001
Output 1
Output 1
Data area Address range
Holding 40001 to 49999
Register
400001 to 465535
Holding 40001 to 49999
Register.
400001 to 465535
Holding 40001 to 49999
Register
400001 to 465535
Data area
to 8192 Output Process Image
to 18192 Input Process Image
to 30512 Input Process Image
to 8192 Output Process Image
to 8192 Output Process Image
CPU DB data area
MB_HOLD_REG
MB_HOLD_REG
MB_HOLD_REG
Programming instructions
6.3 Global library instructions
S7-1200
CPU address
Q0.0 to Q1023.7
I0.0 to I1023.7
IW0 to IW1022
Q0.0 to Q1023.7
Q0.0 to Q1023.7
S7-1200
CPU DB address
Words 1 to 9999
Words 1 to 65535
Words 1 to 9999
Words 1 to 65535
Words 1 to 9999
Words 1 to 65535
199