how to write PDI parameters using S20 Ethernet bus coupler and ctrlX modbus TCP

Open Please see the S20-AI-8 documentation chapter "15.1 Parameter table" for further information. Beware that "Slot-ID" is corresponding to your topology. Further Index is always the object you want to write and data length needs to fit:

*****************************************************************************
* SECTION 2: Startup-Parameterization
*****************************************************************************
* Example for parameterization of IO-modules:
#CMD#
#0x0042# FIBO_PDI_WRITE_OBJECT_REQ
#0x000E# Parameter Count (following words)
#0x0001# Slot-ID
#0x0000# reserved | Subslot
#0x0080# Index
#0x0014# Subindex | Length of following PDI-Data
#0x0002# Byte 0 | Byte 1
#0x0002# Byte 2 | Byte 3
#0x0002# Byte 4 | Byte 5
#0x0002# Byte 6 | Byte 7
#0x0002# Byte 8 | Byte 9
#0x0002# Byte 10 | Byte 11
#0x0002# Byte 12 | Byte 13
#0x0002# Byte 14 | Byte 15
#0x0000# Byte 16 | Byte 17
#0x0000# Byte 18 | Byte 19

See this post above for how to get the file to the bus coupler.