install modbus command line tool

pip3 install modbus_cli

setup udev rules for usb QinHeng Electronics CH340 serial converter

#cat /etc/udev/rules.d/98-usb-serial.rules
KERNEL=="ttyUSB*", ATTRS{idVendor}=="1a86", ATTRS{idProduct}=="7523", OWNER="hass", GROUP="users", MODE="0660", SYMLINK+="rs485"

relay control

# 1
# on 
modbus   -b 9600   -s 2  -v   /dev/rs485  c@0=1
# off
modbus   -b 9600   -s 2  -v   /dev/rs485  c@0=0
# status
modbus -b 9600 -s 2  -v /dev/rs485  c@0/8B | egrep -o '\[.*\]' | tr -d '[,]' | cut -f1 -d' '
# 2
# on 
modbus   -b 9600   -s 2  -v   /dev/rs485  c@1=1
# off
modbus   -b 9600   -s 2  -v   /dev/rs485  c@1=0
# status
modbus -b 9600 -s 2  -v /dev/rs485  c@0/8B | egrep -o '\[.*\]' | tr -d '[,]' | cut -f2 -d' '