Siemens SIMATIC S5 CP 523 Manual page 187

Communications Mode
7.7.3 Receiving Message Frames with 3964(R) Protocol
Basically, data transfers between the CPU and the CP 523 are handled the same way as in
Transparent mode. The additional return information pertaining to the itself is entered in a
coordination byte (CBS, CBR).
CBS = coordination byte "send", CBR = coordination byte "receive".
Schematic for "Receive message frames with the 3964(R) protocol"
1. The CPU forwards a "Coordinate data transfer for receive message frame" request (job
number A080 ) in word 0 of the transfer memory.
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2. The CPU reads out the coordination info from word 0 of the transfer memory, and the CBR
(coordination byte "receive"). A message frame can be received only when the Receive mail-
box contains one (bit 7 of the CBR = "0" when there is no message frame in the Receive
mailbox). Together with the coordination info, the following information is made available to
the user in the transfer memory:
- Word 2: Length (in bytes) of the message frame that is to be received next
- Byte 6: Number of message blocks in the next message frame to be received
- Byte 7: Total number of message frames in the Receive buffer
3. The CPU writes to word 0 of the transfer memory.
The CP updates the transfer memory.
4. The CPU reads the message frame out of the transfer memory in message blocks of 8 bytes
each.
The CPU reads the first message block (8 bytes) out of the transfer memory.
5. The CPU writes to word 0 of the transfer memory.
The CP updates the transfer memory.
6. The CPU reads out the next message block.
7. Steps 5 and 6 are repeated until the CPU has read out the last message block.
7-82
CP 523
EWA 4NEB 811 6044-02a