Alternative to Union from Codesys

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1

Has B&R in Automation Studio with structured text something similar to Union from Codesys?

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2

Hi,

union is not supported right now (maybe with the release of ST OOP, which is coming with AS Code mid of this year).

But if I got it right, it’s more or less a structure where the elements are aligned to the same size, right? So, if declaring a structure with for example 4 byte data types as elements only, it should have the same behavior as described … or I understood the explanation completely wrong, which is possible of course :wink:

Best regards!

3

Hi,

if the challenge is to save memory (e.g. a data base) one can simulate a union with a struct by storing also the data type and using dynamic variables to access the data.

TYPE
	myRecordTypeEnum : 
		(
		REC_DINT,
		REC_REAL
		);
	myRecord : 	STRUCT 
		type : myRecordTypeEnum;
		data : ARRAY[0..3]OF USINT;
	END_STRUCT;
END_TYPE

	IF write_dint THEN
		write_dint := FALSE;
		ddint ACCESS ADR(record.data);
		record.type := REC_DINT;
		ddint := 3;
	END_IF

	IF write_real THEN
		write_real := FALSE;
		dreal ACCESS ADR(record.data);
		record.type := REC_REAL;
		dreal := 3.1415;
	END_IF

	IF read THEN
		read := FALSE;
		CASE record.type OF
			REC_DINT: 
				ddint ACCESS ADR(record.data);
				(* do something with ddint *)
			REC_REAL: 
				dreal ACCESS ADR(record.data);
				(* do something with dreal *)
		ELSE
			(* invalid datatype *)
			
		END_CASE
	END_IF

VAR
	dreal : REFERENCE TO REAL;
	ddint : REFERENCE TO DINT;
	record : myRecord;
	write_dint : BOOL;
	write_real : BOOL;
	read : BOOL;
END_VAR
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4

Not currently as I don’t believe it’s a part of the IEC 61131 main standard. Per this blog post IEC 61131-3: Additional language extensions – Stefan Henneken, twin cat and codesys may support it in some fashion, and per Alex, it might be supported in ST OOP in Automation Studio Code. But currently it isn’t a part of the structured text language as implemented in Automation Studio. Automation Studio’s compliance with the IEC 61131-3 standard is documented in the AS help B&R Online Help. I don’t see it listed as part of the standard.

You may be able to implement something in a library in C to handle this and then wrap it up nicely for structure text usage.

For other’s not familiar, a union defines multiple ways to access the same memory address with different formats. i.e. I can define a UNION of BYTE[4] and a DWORD and then be able to access each of the bytes of the DWORD via the BYTE array without having to do any ACCESS or memcpy(). (It’s a commonly used in C and other languages, but from what I can tell not a part of the 61131-3 standard, edition 3.)

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5

Finally I see someone question about structured text implementation in Automation Studio. :slightly_smiling_face:

I not sure UNION is covered in 61131-3: 2013 but I will say that it is better to have it.
Secondly, I think 61131-3 did covered OOP but ST in B&R Automation Studio does not have it. I glad to hear that we will get it soon(ST OOP).

6

Thank you for all the replies. I use union mostly for conversions, for example, Modbus registers (status and control words) and PLC internal flags. I think Matt Buck’s approach with the C function will help me.

7

this is called ‘type-punning’ and there is a nice article on stackoverflow about it:

TLDR: Using Unions for “type punning” is fine in C, and fine in gcc’s C++ as well (as a gcc [g++] extension). But, “type punning” via unions has hardware architecture endianness considerations

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