BECKHOFF C1220 - Manual (Page 21)

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21 Software description of functions
Beckhoff Lightbus - PC interface card C1220
Before the interrupts are activated, a read access must take place to the
cell IRQ inputs in the General Control Block (see Chapter 3.5).
String Communication
General
String communication is used for packet oriented data exchange with
peripheral modules. Usually, parameter data is exchanged with the
modules (e.g. parameterisation of a BK2000 by register interface).
This type of communication also permits slave to slave communication, as
well as communication between master and slave. The master card then
functions simply as a relay station.
The following resources are required to carry out string communication:
– 2 CDLs for sending or receiving the strings.
– 2 buffers in DPRAM for string storage, the buffer size being
parameterisable.
String structure
A data string consists of a four-byte string header and a string data area.
The header contains the necessary routing information, and the data area
contains the user data itself. The entire string can have a maximum length
of 255 bytes.
A string has the following structure:
Offset Description
0x00 Address of the sender (TX)
0x01 Address of the receiver (RX)
0x02 Channel / priority (only relevant to the BK2000)
0x03 String length
0x04
... String data
0xFF