How to use function code 0x06 to read from register?

[ruteclrp]

I have a heatpump where the implementation of Modbus appear somewhat unconvential. Case is that for each register type (function code) the starting address is identical.
For example - for function code 0x04:
Request:
Function code 1 byte 0x04
Starting address 2 bytes 0x0001 – 0x0014 (20)
Quantity of input registers 2 bytes 0x0001
Response:
Function code 1 byte 0x04
Byte count 1 byte 0x02
Sensor value registers 2 bytes 0xHiLo (sensor values 1-20)

For function code 0x06:
Request:
Function code 1 byte 0x06
Register address 2 bytes 0x0001 – 0x00D2 (*1)
Register value 2 bytes 0x0000 – 0x00FF (*2)
Response:
Function code 1 byte 0x06
Register address 2 bytes 0x0001 – 0x00D2
Register value 2 – 4 bytes Depending on msg. See list of variables.
(*1) Add 0x0100 to Register address for writing request (only for address 0x0001 – 0x0091 )
(*2) depending on each address min/max area. Value is Do-Not-Care on reading only.

I can write using the function code 0x06 by adding 0x0100 (256) to each register address.

I can however not read the content of the registers under function code 0x06, as that requires using the write function to the lower address space and ignoring the write payload and rather take the response values and treat them as if I was using function code 0x03 or 0x04.

How could I possibly read those addresses and process the response using the write function 0x06?

appreciate anyone with deeper insight in pymodbus.

[janiversen]

There are a lot of devices that do not have shared address space, so you treat the 4 different blocks as individual address spaces. Please see the simulator datastore simulator documentation where this is explained.

I am not sure I understand your problem, especially not the mix with write.

Seems your device uses a different address to write than to read, that is somewhat very old fashioned, but works....you cannot read the "write address" you read the lower address and write the upper address...when writing the result should be in the lower address (depending on the type of register, e.g. set_temperature, write sets the temperature but read reads the actual temperature.

This is not a pymodbus problem, but how to handle your device, and that is something you need to read in the manual of the device.

[janiversen]

And just for the record 0x06 can pr definition not READ...it's a write function.

[janiversen]

Just checked pymodbus delivers the register value in the return, when using 0x06, so no need for tweaking seen from pymodbus pow.

I think the manual tries to say if you issue 0x06, there are no write but the values is returned as normal..,,so no breaking of the modbus protocol.

[ruteclrp]

Thanks a lot. I know the response I wrote was a bit sloppy - however it was more to indicate what I was talking about. Naturally, the register is 2 bytes, and the value also 2 byte.

Anyway, just to understand your response. I read it as you are proposing adding a new "read" method which build on e.g. 0x04 calls but uses 0x06 in the execution of the "read" call.
Ideally I would need an additional input_type in HA, such as "input_6" which would work just as input type "input" but call using 0x06 rather than 0x04. I wonder how difficult that would be. In any case, in HA I would need some way to tell the integration that the "read" call should use function 0x06.

To be perfectly honest here, I am in deep water here :-)

[ruteclrp]

So, on your second reply above, it is really the parsing of the returned register value within HA which is the issue, and possibly, adding a new "input_6" method to the input type options could make it possible for the standard "input" method to just used 0x06 rather than 0x04 in the call.

[janiversen]

well at least it is not a pymodbus issue….and in HA it is a lot more than just adding a new input_type, it’s tweaking a fair amount of code.

It is probably easier to tweak the read request/response in pymodbus and keep HA unchanged. Or even easier, make a new library that sits between pymodbus and HA that does the tweaking you need…anyhow it is up to you.