Modbus Addressing - Siemens SIMATIC S7-200 System Manual

S7-200 Programmable Controller System Manual

Modbus Addressing

Modbus addresses are normally written as 5 character values containing the data type and the
offset. The first character determines the data type, and the last four characters select the proper
value within the data type.
Modbus Master Addressing -- The Modbus Master instructions then map the address to the
correct functions to send to the slave device. The following Modbus addresses are supported by
the Modbus Master instructions:
00001 to 09999 are discrete outputs (coils)
-
10001 to 19999 are discrete inputs (contacts)
-
30001 to 39999 are input registers (generally analog inputs)
-
40001 to 49999 are holding registers
-
All Modbus addresses are one-based, meaning that the first data value starts at address one. The
range of valid addresses will depend on the slave device. Different slave devices will support
different data types and address ranges.
Modbus Slave Addressing --The Modbus Master device then maps the addresses to the correct
functions. The following addresses are supported by the Modbus Slave instructions:
00001 to 00128 are discrete outputs mapped to Q0.0 - Q15.7
-
10001 to 10128 are discrete inputs
-
mapped to I0.0 - I15.7
30001 to 30032 are analog input
-
registers mapped to AIW0 to AIW62
40001 to 04xxxx are holding
-
registers mapped to V memory.
All Modbus addresses are one-based.
Table 12-1 shows the mapping of Modbus
addresses to the S7-200 addresses.
The Modbus Slave Protocol allows you to
limit the amount of inputs, outputs, analog
inputs, and holding registers (V memory)
accessible to a Modbus master.
The MaxIQ parameter of the MBUS_INIT
instruction specifies the maximum number
of discrete inputs or outputs (Is or Qs) the
Modbus master is allowed to access.
The MaxAI parameter of the MBUS_INIT
instruction specifies the maximum number
of input registers (AIWs) the Modbus
master is allowed to access.
The MaxHold parameter of the MBUS_INIT
instruction specifies the maximum number
of holding registers (V memory words) the
Modbus master is allowed to access.
See the description of the MBUS_INIT
instruction for more information on setting
up the memory restrictions for the Modbus
slave.
350
Table 12-1 Mapping Modbus Addresses to the S7-200
Modbus Address
00001
00002
00003
...
00127
00128
10001
10002
10003
...
10127
10128
30001
30002
30003
...
30032
40001
40002
40003
...
4xxxx
S7-200 Address
Q0.0
Q0.1
Q0.2
...
Q15.6
Q15.7
I0.0
I0.1
I0.2
...
I15.6
I15.7
AIW0
AIW2
AIW4
...
AIW62
HoldStart
HoldStart+2
HoldStart+4
...
HoldStart+2 x (xxxx- -1)