feat(cmd/modbus-cli): support writing single coil

[frzifus]

With this pr the modbus-cli tool is extended by the function 0x05. This allows to switch individual outputs on and off.

Usage:

# read
modbus-cli -slaveID=0xFF -address=tcp://127.0.0.1:502 -fn-code=0x01 -type-parse=uint8 -register=54321 -quantity=1 -log-frame

# Turn on
modbus-cli -slaveID=0xFF -address=tcp://127.0.0.1:502 -fn-code=0x05 -type-parse=uint8 -register=54321 -quantity=1 -log-frame -write-value=0xFF00

# Turn off
modbus-cli -slaveID=0xFF -address=tcp://127.0.0.1:502 -fn-code=0x05 -type-parse=uint8 -register=54321 -quantity=1 -log-frame -write-value=0x0000

Ref: https://ozeki.hu/p_5880-mobdbus-function-code-5-write-single-coil.html

Signed-off-by: Benedikt Bongartz benne@klimlive.de

[andig]

IMHO the point here is that the on/off values for the coils are non-obvious. Do they depend on modbus implementation's endianess? If not, the on/off values should be part of the api.

[guelfey]

I agree, I'd rather not have to know this detail that on is 0xFF00 (I had to look this up right now). Maybe just take any nonzero value as on?

[frzifus]

Do they depend on modbus implementation's endianess?

oh good question, will try to get that out.

Maybe just take any nonzero value as on?

Sounds fair to me.

[andig]

What is missing to get coils support merged?

[frzifus]

öhm, seems we're almost celebrating a one year anniversary. :partying_face: :balloon:

Do they depend on modbus implementation's endianess?

This is what modbus.org says about it:

4.2 Data Encoding • MODBUS uses a ‘big-Endian’ representation for addresses and data items. This means that when a numerical quantity larger than a single byte is transmitted, the most significant byte is sent first. So for example Register size value 16 - bits 0x1234 the first byte sent is 0x12 then 0x34

Other than that, i haven't found any further info during the last year.

Maybe just take any nonzero value as on?

Adapted it, now every value that is non-zero represents on.

[andig]

Recently used https://ozeki.hu/p_5880-mobdbus-function-code-5-write-single-coil.html. Seems we can acceppt any non-zero Value as "on", but devices will only ever accept/return 0xFF00 bigendian.