Siemens SIMATIC S5 CP 523 Manual page 169

Communications Mode
7.6.1 Sending Message Frames with the Message Frame Length Specification
You specify the message frame length in bytes with the Send request in word 2.
Note:
The length of the message frame sent by the CPU can vary from the length configured in
parameter block 7 ( 7.3). The length configured in parameter block 7 refers to message frames
received by the CP from a peripheral device.
Procedure for "Send message frames with message frame length specification"
1. The CPU transfers the following in the transfer memory
• The message frame length in bytes in word 2
• The "Coordinate data transfer for sending message frames" job number into word 0.
2. The CPU reads the coordination information from word 0 of the transfer memory.
If the CP has not accepted the job (bit 0 in byte 1= "0"), no message frame can be sent.
The CPU can read the number of message blocks to be sent from byte 6 of the transfer
memory. The number is calculated by the CP using the length specified in the job request. If
you specify a message frame length of 12 bytes, for example, the CP stores the value 02
byte 6 for two message blocks.
3. CP transfers message frame in 8-byte message blocks to the transfer memory.
Note:
The CP stores the number of message frames in the Receive mailbox in byte 7, so you can also
determine after issuing a Send request whether you can receive a message.
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CP 523
in
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EWA 4NEB 811 6044-02a