Mapping the PLC and IEC process data

The TwinCAT PLC process data are cyclically mapped (copied) into the IEC process data (application objects) and vice versa at program runtime. Up to four process data ranges (IO inputs, IO outputs, flag range, data range) can be declared in the PLC program as buffer variables for the mapping of the IEC<->PLC process data. The byte size of the buffers is freely selectable and may be different for each area. Unused ranges need not necessarily be declared.

In our introductory example we declare 4 PLC process data areas with 3000 bytes each:

PROGRAM MAIN
VAR
AODB : ARRAY[0..49] OF ST_IEC870_5_101AODBEntry;
hTable : T_HAODBTable;

init : BOOL := TRUE;
initError : UDINT;
asduAddr : UDINT := 7;


inputs AT%IB0 : ARRAY[0..2999] OF BYTE;
outputs AT%QB0 : ARRAY[0..2999] OF BYTE;
memory AT%MB0 : ARRAY[0..2999] OF BYTE;
data : ARRAY[0..2999] OF BYTE;

END_VAR

How the process data are to be mapped at runtime is specified during configuration of the application objects via the F_iecAddTableEntry function.

See also in: Definition and configuration of application objects.

The buffer variables were now declared as byte arrays. In order to improve access to the required data we define the individual variables a second time and allocate them to the corresponding byte/bit offset addresses. In case of a change in the byte array, the corresponding individual variable will be changed and inverted at the same time. However, this is not absolutely necessary. The bytes/bits of the byte array buffer variables can be accessed directly.

VAR_GLOBAL(* Memory offset 0..99 unused *)
(* Single points *)
msgSingle_0 AT%MX100.0 : BOOL;
msgSingle_1 AT%MX100.1 : BOOL;
msgSingle_2 AT%MX100.2 : BOOL;

(* Double points *)
(* Bit 0..1 = first double point,
Bit 2..3 = second double point, 
Bit 4..5 = third double point, 
Bit 6..7 = fourth double point *)
msgDouble_0 AT%MB200 : BYTE;

(* Regulating step values *)
msgStep_0 AT%MB300 : BYTE;
msgStep_1 AT%MB301 : BYTE;
msgStep_2 AT%MB302 : BYTE;

(* 32 bit strings *)
msgBitStr_0 AT%MD400 : DWORD;
msgBitStr_1 AT%MD404 : DWORD;
msgBitStr_2 AT%MD408 : DWORD;

(* Measured values, normalized values *)
msgNormalized_0 AT%MW500 : WORD;
msgNormalized_1 AT%MW502 : WORD;
msgNormalized_2 AT%MW504 : WORD;

(* Measured values, scaled values *)
msgScaled_0 AT%MW600 : INT;
msgScaled_1 AT%MW602 : INT;
msgScaled_2 AT%MW604 : INT;

(* Measured values, short floating point values *)
msgFloating_0 AT%MD700 : REAL;
msgFloating_1 AT%MD704 : REAL;
msgFloating_2 AT%MD708 : REAL;

(* Integrated totals *)
msgTotal_0 AT%MD800 : UDINT;
msgTotal_1 AT%MD804 : UDINT;
msgTotal_2 AT%MD808 : UDINT;

(* Single commands *)
cmdSingle_0 AT%MX2100.0 : BOOL;
cmdSingle_1 AT%MX2100.1 : BOOL;
cmdSingle_2 AT%MX2100.2 : BOOL;

(* Double commands *)
(* Bit 0..1 = first double command,
Bit 2..3 = second double command, 
Bit 4..5 = third double command, 
Bit 6..7 = fourth double command *)
cmdDouble_0 AT%MB2200 : BYTE;


(* 32 bit string commands *)
cmdBitStr_0 AT%MD2400 : DWORD;
cmdBitStr_1 AT%MD2404 : DWORD;
cmdBitStr_2 AT%MD2408 : DWORD;

(* Set point, normalized values *)
cmdNormalized_0 AT%MW2500 : WORD;
cmdNormalized_1 AT%MW2502 : WORD;
cmdNormalized_2 AT%MW2504 : WORD;

(* Set point, scaled values *)
cmdScaled_0 AT%MW2600 : INT;
cmdScaled_1 AT%MW2602 : INT;
cmdScaled_2 AT%MW2604 : INT;

(* Set point, short floating point values *)
cmdFloating_0 AT%MD2700 : REAL;
cmdFloating_1 AT%MD2704 : REAL;
cmdFloating_2 AT%MD2708 : REAL;
END_VAR

Mapping of the IEC<->PLC process data in the central station

Process data in the monitoring direction (Slave->Master information)

Example 1

Single point information (M_SP_NA_1) with the IOA 100, PLC flag range, byte offset = 100, bit offset = 0.

Substation -> ... -> Central Station FB -> Memory[100].0 == msgSingle_0

Example 2

Measured value, short floating point value (M_ME_NC_1) with the IOA 700, PLC flag range, byte offset = 700, bit offset = 0 (no meaning).

Substation -> ... -> Central Station FB -> Memory[700 to 703] == msgFloating_0

Process data in the control direction (Master->Slave commands)

Example 1

Single command state (C_SC_NA_1) with the IOA 10, PLC flag range, byte offset = 2100, bit offset = 0.

cmdSingle_0 == memory[2100].0 -> Central Station FB -> ... -> Substation

Example 2

Set point, short floating point value (C_SE_NC_1) with the IOA 70, PLC flag range, byte offset = 2700, bit offset = 0 (no meaning).

cmdFloating_0 == memory[2700 to 2703] -> Central Station FB -> ... -> Substation