Modbus: how to read strings?

Add-ons Bindings
1

I have a modbus device which stores a string in consecutive holding registers which i’m trying to read.

Right now, I would configure a modbus slave to read 8 unit16 values (for a 16 char string). How do I go about reading those registers and putting the result into a string?

Obviously

String mystringhere “[%s]” {modbus=“slave:0”}

will not work.

2

The best idea I would come up at the moment would be to read 16 individual int8 bytes and write a rule to put these into a string variable.
Can you imagine reading 10 of those strings, though? 160 items?

3

Yup. Modbus was never intended to handle strings.

You could log an enhancement request for the binding, I guess that would be for known fixed-length strings, packed into ASCII bytes? Any other sensible options? What should happen about writes?

4

Hmm, ideally, one would define a valuetype=string or char in the slave definitions and simply read the number of characters into a string item.

writes could work the same way, with padding ether 0x0 or 0x20 (space) if the string item holds fewer characters than the slave definition specifies.

5

Well, there are different character encodings, as well as the perennial big-endian/little-endian word split debate, if the characters are packed. And non-packed options, for a char per word.
And a padding choice for writes, as you pointed out.

Length would be covered by existing config parameter.

Someone would come up with a requirement for that be a maximum length, and terminate string building when encountering a control character, e.g. CR or null. Some might want to include control chars, others strip them - but I think the new transformation capability would take care of that?

Certainly worth making the enhancement request with a real use-case, that should go forward to OH2 binding even if it never happens for OH1.

6

@ssalonen are there plans to implement string types in the new binding?

7

Hi @mal

It’s completely doable and there’s actually even some core functionality there to support reading strings from registers. Wouldn’t be too big deal actually.

Currently, personally I have no plans to work on this but I welcome all contributions :slight_smile:

Best,
Sami