Siemens SIMATIC S7 Manual page 20

Adaption of the
Modbus
Parameters
server_client
single_write
MODBUS / TCP communication via the integrated PN interface of the CPU
2XV9450-1MB02; Manual edition 2.1
The Modbus parameters in the block MODBUS_PARAM define the mode of
operation of the Modbus communication and the address reference of
Modbus addresses and SIMATIC addresses.
TRUE: S7 is server
FALSE: S7 is client
In operating mode "S7 is client" and single_write = TRUE write requests with
length 1 are carried out with the function codes 5 and 6.
With single_write = FALSE all write requests use the function codes 15 and
16.
Parameterisation of the Modbus Communication
4-6