Communication between TwinCAT and Bus Terminal Controller

  BECKHOFF Fieldbus Components: Programming

Communication between TwinCAT and Bus Terminal Controller

It makes sense to define a data structure for transferring data between TwinCAT and Bus Terminal Controller. Please note the following to account for the differences in data management on the two systems.

• If two different data types are sent in sequence (e.g. byte and INT), the following variable is set to the next even address offset
• Boolean variables should never be allocated individually within a structure, since they would invariably occupy 1 byte. Boolean expressions should always be masked in a byte or word.

Example 1: Structure on the BX and on the PC

Variable BX memory PC memory (TwinCAT) Byte %..B0 %..B0 INT (1) %..B2 %..B1 INT (2) %..B4 %..B3

Because the first byte is followed by a different variable type (INT), it was set to the next free even address in the BX/BCxx50. In order to achieve the same data structure on both systems, a dummy byte has to be inserted in the PC project (see example 2).

Example 2: Structure on the BX and on the PC with identical memory allocation

Variable BX memory PC memory (TwinCAT) Byte %..B0 %..B0 Byte
(Dummy) %..B1 (not necessarily required, since the system deals with this itself if the variable does not exist) %..B1 INT (1) %..B2 %..B2 INT (2) %..B4 %..B4

Data structure

Type PB_Data
STRUCT
    wVar_1:WORD;
    iValue_1:INT;
    iValue_2:INT;
    iValue_3:INT;
END_STRUCT
END_TYPE

Creating a variable structure

VAR_Global
    strData_Out AT %QB1000:PB_Data; (*PLC Variables *)
    bInput_01 AT %IX0.0:BOOL; (* Input from a terminal *)
END_VAR

Small programming example

strData_Out.wVar_1.0:=bInput_01;

Note A mixed data structure should not contain any Real values. If it does, the High and Low word must be swapped in the BX/BCxx50or in the TwinCAT master project. It is better to use an array of Real values or to transfer the Real values individually. Note It is also possible to transfer larger fieldbus data blocks, in order to leave a reserve for your structure. Disadvantage: These reserves are then transferred with each fieldbus telegram, causing additional load for the fieldbus communication.