Siemens SIMATIC S7 System Manual page 474

Libraries
9.2 Overview of Modbus communication
Mapping Modbus addresses to CPU addresses
All Modbus addresses are one-based.
Table 9- 1
MBUS_INIT parameters that limit slave accessibility
The Modbus slave/protocol allows you to limit the number of inputs, outputs, analog inputs,
and holding registers (V memory) that are accessible to a Modbus master.
● MaxIQ assigns the maximum number of discrete inputs or outputs (Is or Qs) a Modbus
master is allowed to access.
● MaxAI assigns the maximum number of input registers (AIWs) a Modbus master is
allowed to access.
● MaxHold assigns the maximum number of holding registers (V memory words) a Modbus
master is allowed to access.
See the description of the MBUS_INIT (Page 491) instruction for more information on setting
up the memory restrictions for the Modbus slave.
474
Mapping Modbus addresses to CPU addresses
Modbus address
00001
00002
00003
...
00255
00256
10001
10002
10003
...
10255
10256
30001
30002
30003
...
30056
40001 400001
40002 400002
40003 400003
...
4yyyy 4zzzzz
CPU address
Q0.0
Q0.1
Q0.2
...
Q31.6
Q31.7
I0.0
I0.1
I0.2
...
I31.6
I31.7
AIW0
AIW2
AIW4
...
AIW110
Vx (Holding reg. start)
Vx+2 =(Hold reg. start+2)
Vx+4 =(Hold reg. start+4)
...
Vx+2(yyyy-1) or Vx+2(zzzzz-1)
System Manual, V2.3, 07/2017, A5E03822230-AF
S7-200 SMART