Siemens SIMATIC S5 CP 523 Manual page 175

Communications Mode
7.6.3 Sending Message Frames with 3964(R) Protocol
In principle, data transfers between the CPU and the CP 523 are handled the same way as in
Transparent mode. The additional return information about the protocol itself is entered in a so-
called coordination byte (CBS, CBR).
CBS = Coordination byte 'send', CBR = coordination byte 'receive'.
You specify the message frame length in word 2 when you submit the request.
"Send message frames with the 3964(R) protocol"
1. The CPU forwards the following in the transfer memory:
• The message frame length in bytes in word 2
• The job number for "Coordinate data transfer for sending message frame" in word 0.
2. The CPU reads the coordination info out of word 0 of the transfer memory.
No message frame can be transmitted if the CP rejects the request (bit 0 in byte 1="0").
Together with the coordination info, the following is made available to the CPU:
• The coordination byte 'send' (CBS), including error flags, in byte 2
• The number of message blocks comprising the frame in byte 6
On the basis of the length specification given in the job request, the CP automatically
computes the number of message blocks in the frame to be transmitted.
If, for example, you specified a message frame length of 12 bytes, the CP enters the value 02
in byte 6, i.e. 2 message blocks.
3. The CPU transfers the message frame in 8-byte message blocks to the transfer memory.
Note:
The CP enters the number of message frames in the Receive mailbox in byte 7, thus making it
possible for you to ascertain whether you can receive a message frame even after a Send request
has been submitted.
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CP 523
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EWA 4NEB 811 6044-02a