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Communications Mode CP 523

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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Siemens KS7 Series Spezifikationen Seite 169