How to access a nested structure using PyADS

Question

I have two structures defined in TwinCAT3 like so:

TYPE ADSUWDisplay :
STRUCT
    sYarn : STRING;
    sNextYarn : STRING;
    lLength : REAL;
    iPosition : INT;
    sYarnSpec : STRING;
    iPackageCount : INT;
    iPackageTotalCount : INT;
    iCount : INT;
    iTotalCount : INT;
    sUpcomingYarn : STRING;
    sUpcomingYarnSpec : STRING;
    sUWMessage : STRING;
    sThreadUp : STRING;
END_STRUCT
END_TYPE

TYPE ADSRemoteDisplay :
STRUCT
    iUW : INT;
    iCount : INT;
    sState : STRING
    iStateNo : INT;
    sRobotMessage : STRING;
    adsUWDisplay : ARRAY[0..5] OF ADSUWDisplay;
END_STRUCT
END_TYPE

As you can see, ADSUWDisplay is nested inside ADSRemoteDisplay.

How can I access ADSUWDisplay using PyADS?

More specifically, how do I declare my Structure defs for use in pyads.connection.read_structure_by_name()?

Please help!

Thanks

score 0 · Answer 1 · answered Oct 20 '22 at 09:36

You can read out a struct as follows, from the pyads docs:

Use the parameter structure_def to define the structure and array_size to define the size of the array.

 >>> structure_def = (
         ("i", pyads.PLCTYPE_INT, 1),
         ("s", pyads.PLCTYPE_STRING, 1)
     )
 >>> symbol = plc.get_symbol("MyStructure", structure_def=structure_def, array_size=2)
 >>> symbol.write([{"i": 1, " "s": "foo"}, {"i": 2, "s": "bar"}])
 >>> symbol.read()
[{"i": 1, " "s": "foo"}, {"i": 2, "s": "bar"}]

Not sure how to define a nested struct. I guess you can try to add a nested tuple in structure_def, e.g.:

structure_def = (
         ("i", pyads.PLCTYPE_INT, 1),
         (
             ("s1", pyads.PLCTYPE_STRING, 1),
             ("s2", pyads.PLCTYPE_STRING, 1)
         )
     )

How to access a nested structure using PyADS

1 Answers1