Siemens SIMATIC S7 System Manual page 490

Libraries
9.3 Modbus RTU library
The Modbus slave instructions support the Modbus RTU protocol. These instructions use the
Freeport feature of the S7-200 SMART CPU to support the most common Modbus functions.
The following Modbus functions are supported:
Function
1
2
3
4
5
6
15
16
490
Description
Read single/multiple coil (discrete output) status. Function 1 returns the on/off status of
any number of output points (Qs).
Read single/multiple contact (discrete input) status. Function 2 returns the on/off status
of any number of input points (Is).
Read single/multiple holding registers. Function 3 returns the contents of V memory.
Holding registers are word values under Modbus and allow you to read up to 120 words
in one request.
Read single/multiple input registers. Function 4 returns analog Input values.
Write single coil (discrete output). Function 5 sets a discrete output point to the specified
value. The point is not forced and the program can overwrite the value written by the
Modbus request.
Write single holding register. Function 6 writes a single holding register value to the V
memory of the S7-200 SMART.
Write multiple coils (discrete outputs). Function 15 writes the discrete output values to
the Q image register of the S7-200 SMART. The starting output point must begin on a
byte boundary (for example, Q0.0 or Q2.0) and the number of outputs written must be a
multiple of eight. This is a restriction for the Modbus slave protocol instructions. The
points are not forced and the program can overwrite the values written by the Modbus
request.
Write multiple holding registers. Function 16 writes multiple holding registers to the V
memory of the S7-200 SMART. There can be up to 120 words written in one request.
System Manual, V2.3, 07/2017, A5E03822230-AF
S7-200 SMART