Siemens SIMATIC S5 CP 523 Manual page 165

Communications Mode
7.6 Sending Message Frames to a Peripheral Device (Job Number A001
The CPU can forward up to 256 bytes to the CP 523 with one Send request. This data is referred to
as a message frame in the following.
Schematic for " sending a message frame"
S5-CPU
Send message
frame
Figure 7-7. Sending Message Frames to a Peripheral Device
Data transfer between the CPU and the CP is always initiated by the CPU with a job request for
"Coordinate data transfer for sending a message frame". When the CP has accepted the send
job, the CPU transfers the message frame in message blocks of eight bytes.
The CP 523 has a Send mailbox of 256 bytes for storing the data from the CPU. The CP 523
accepts data from the transfer memory in the Send mailbox if the CPU has written the
relevant data into word 0 of the transfer memory (
After the CP has received the complete message frame, it transfers the message frame
autonomously from the Send mailbox to the peripheral device via the serial interface (
The CP can only accept another Send request when it has transferred the complete message
frame to the peripheral device.
The following two pages contain a graphic representation of this procedure:
• "Send message frame" flowchart
• Transfer memory assignments from the user program and from the CP 523 for servicing the
Send request
Note:
See 8.1 for a printout and explanation of the statement list of the "SEND" function block. The
function block is a user-friendly interface for handling "Send message frame" requests.
Use of the function block is also explained in 8.1.
7-60
CP 523
8-byte
transfer
memory
Internal bus
Serial
interface
Send
mailbox
256 bytes
).
EWA 4NEB 811 6044-02a
CP 523
)
H
Peripheral
device
Receive
message
frame
).