petervg@linuxcnc1:~$ mbpoll -b9600 -Pnone -a1 -t0 -r20 -c1 /dev/ttyUSB0
mbpoll 1.0-0 - FieldTalk(tm) Modbus(R) Master Simulator
Copyright © 2015-2019 Pascal JEAN, https://github.com/epsilonrt/mbpoll
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.

Protocol configuration: Modbus RTU
Slave configuration...: address = [1]
                        start reference = 20, count = 1
Communication.........: /dev/ttyUSB0,       9600-8N1 
                        t/o 1.00 s, poll rate 1000 ms
Data type.............: discrete output (coil)

-- Polling slave 1... Ctrl-C to stop)
[20]: 	0
-- Polling slave 1... Ctrl-C to stop)
[20]: 	0
-- Polling slave 1... Ctrl-C to stop)
[20]: 	0
-- Polling slave 1... Ctrl-C to stop)
[20]: 	0
-- Polling slave 1... Ctrl-C to stop)
[20]: 	0