BECKHOFF BC3150 - Manual (Page 64)

Safety Instructions
62 Fieldbus Components
Communication between TwinCAT and BX/BCxx50
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 oc-
cupy 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.