Hi Michael
to check the connection try to read values from the device (e.g. flow rate or a temperature). Writing is a bit more complicated.
I did my first tests with modpoll (under linux, mabe it is also available for windows)
Reading the supply air temperature:
[19:45:16] oh@openHABianPi2:~/mbpoll/build$ mbpoll -v -m rtu -a 11 -r 4009 -c 1 -b 9600 -d 8 -p even -1 /dev/ttyUSB0
debug enabled
Set mode=rtu
iGetIntList(11)
Integer found: 11
iCount=1
Set start reference=4009
Set number of values=1
Set rtu baudrate=9600
Set rtu data bits=8
Set device=/dev/ttyUSB0
mbpoll 1.4-8 - FieldTalk™ Modbus(R) Master Simulator
Copyright © 2015-2018 Pascal JEAN, GitHub - epsilonrt/mbpoll: command line utility to communicate with ModBus slave (RTU or TCP)
This program comes with ABSOLUTELY NO WARRANTY.
This is free software, and you are welcome to redistribute it
under certain conditions; type ‘mbpoll -w’ for details.
Opening /dev/ttyUSB0 at 9600 bauds (E, 8, 1)
Set response timeout to 1 sec, 0 us
Protocol configuration: Modbus RTU
Slave configuration...: address = [11]
start reference = 4009, count = 1
Communication.........: /dev/ttyUSB0, 9600-8E1
t/o 1.00 s, poll rate 1000 ms
Data type.............: 16-bit register, output (holding) register table
-- Polling slave 11...
[0B][03][0F][A8][00][01][06][54]
Waiting for a confirmation...
<0B><03><02><00><1F><61><8D>
[4009]: 31
31 meaning 3.1°C
If this doesn’t work, as rossko57 said, double check the communication parameters: baudrate, parity, stop bits, modbus slave address.
You max also check the status of your open hab items: Are they online? Have you checked to openhab logs? You can increase the log level of the modbus plugin to get more detailed information about the problem. There is some documentation about how to increase the logging in OH. IIRC check the modbus plugin docu, it should state about how to increase the log level for diagnostics.
br
Bernd