Siemens SIMATIC S5 Manual page 86

Data Transmission Using a Standard Connection
Principle of Operation
For this type of communication you need to define the following once:
• The send mailbox (SF) that contains the transmit data (maximum 242 bytes).
• The send coordination byte (CBS) that coordinates between user program and SINEC L2.
• The receive mailbox (RM) that contains the received data (maximum 242 bytes).
• The receive coordination byte (CBR) that coordinates between user program and SINEC L2.
The coordination bytes are stored in the flag area. The send and receive mailboxes are usually
stored in a data block. Figure 4-2 illustrates a standard connection.
Source PLC
Active network station
Byte 0
Byte 2
Byte 3
.
.
.
.
Byte 243
Bus
*
Send_Erl = bit "transmit allowed"
Figure 4-2. Functional Diagram of a Standard Connection
Explanations to Figure 4-2:
If you want to transmit a message, write to the send mailbox: Enter in the send mailbox the amount
of data to transmit (amount of net data) and the receiver's address (L2 destination address), and
then store the message (net data). Then grant the permission to transmit in send coordination byte
(set bit "Send_Erl").
The S5-95U sends the message to the addressed receiver via the bus.
After the message has arrived at the receive mailbox, bit "Empf_Erl " (receive allowed) is reset in
the receive coordination byte. The received data are available for evaluation in the receive mailbox.
To restore the receive readiness, empty the receive mailbox, and set bit "Empf_Erl " in the receive
coordination byte.
4-2
S5-95U
Send coordination
byte (CBS)
(in flag area)
Send_Erl * Error
Send mailbox (in flag or
data area)
Amount of net data
L2 destination address
1 to 242 bytes
net data
Empf_Erl = bit "receive allowed"
Destination PLC
Passive or active network station
Receive coordination
byte (CBR)
(in flag area)
Empf_Erl *
Receive mailbox(in flag
or data area)
Byte 0
Amount of net data
Byte 2 L2 source address
Byte 3
.
1 to 242 bytes
.
net data
.
.
Byte 243
S5-95U, SINEC L2
S5-95U
Error
EWA 4NEB 812 6112-02