Data exchange between A4 and A5/A6 controllers with CODESYS V2

Apply for CODESYS V2 and affects the data transfer of structures with different elements (for example BOOL, WORD …) between A4 and A5/A6 controller via TCP/IP, UDP or CODESYS network functions.

 

Different memory alignment

After compiling and transferring plc programms to the controller, variables occupy internal memory of the controller:

Type of variable	Minimum memory requirement in the internal memory
BOOL	1 bytes
BYTE	1 bytes
WORD	2 bytes
DWORD	4 bytes
REAL	4 bytes
LREAL 1)	8 bytes

1)	LREAL variables must not be exchanged between A4 and A5/A6. The variable type LREAL is not supported in A4. Siehe 'PLC projects from A5/A6 used on A4 controllers'.

 

For structure variables, several variables are combined into data structures. These structures occupy different areas (compiler-dependent) in the internal memory of a controller. The programming software CODESYS uses different compilers for the controllers A4 and A5/A6, which results that the structure variables being stored differently in the internal memory.

 

The A5/A6 controller compiler stores data in succession in the internal memory, regardless of the data type.

The A4 controller compiler works according to the following rules:

• A variable from the WORD type is in an internal memory area whose byte index is divisible by 2 without a remainder.
• A variable from the DWORD type is in an internal memory area whose byte index is divisible by 4 without a remainder.
• A variable from the REAL type is in an internal memory area whose byte index is divisible by 4 without a remainder.
• A variable from the BYTE type is stored in any internal memory.
• A type BOOL variable occupies one byte and behaves like the BYTE type.

 

Example:

An assumed structure from an A5/A6 controller should be transferred to an A4.

Exemplary assumed structure variable:

Structure content		Internal memory
Variable name	Type of variable	Memory requirement in the internal memory
boVar1	BOOL	1 bytes
dwVar2	DWORD	4 bytes
byVar3	BYTE	1 bytes
wVar4	WORD	2 bytes

 

Alignment in the internal memory [bytes]	0	1	2	3	4	5	6	7	8	9	10	11
A5/A6 controller	boVar1	dwVar2				byVar3	wVar4
A4 controller	boVar1				dwVar2				byVar3		wVar4

 

As long as structure variables are not exchanged between different controllers, the internal memory allocation is not relevant for the user.

If data is exchanged between the controllers A4 and A5/A6, the transmitted data in the receiver's internal memory is stored as it was sent. However, the receiver's compiler expects the data in the above-illustrated arrangement, which causes the transmitted data in the receiver to be incorrectly assigned to the structure variables. In the previous example, only the variable 'boVar1' in the receiver unit would be assigned the correct content.

 

Proposals for solutions, shown by example

The data in the transmitter must be arranged so that it can also be correctly assigned in the memory alignment of the receiver.

 

 

 

The structure expanded with the dummy variables can be correctly transmitted from an A5/A6 controller to an A4 controller and be correctly assigned to the structure variables by the A4 compiler. The transmission of data from an A4 to an A5/A6 controller also works with the expanded structure variable.