Rockwell Automation DAG6.5.8 APPLICATION GUIDE SCADA SYSTEM - Manual (Page 34)

Publication AG-UM008C-EN-P - February 2005
1-16 Designing Communication
Designing Communication
for DF1 Full-Duplex
Protocol
When designing communication using DF1 full-duplex protocol, you
must configure timeout values and retry counts that control the
communication between a transmitting station and a receiving station.
Consider the type of link media you are using to help you determine
the best values for the timer and counters. For example, you can
expect a message being sent over a satellite link to take longer than
one being sent over a telephone leased-line link. Figure 1.8 shows the
communication sequence for DF1 full-duplex protocol.
Figure 1.8 Read or Write Requests via DF1 Full-Duplex
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•
Ladder logic
triggers MSG
•
DF1 driver
formats
command
packet
•
Send command
packet
•
Start ACK timer
•
Wait for ACK
(or timeout)
•
Start reply
timer
•
Wait for
reply (or
timeout)
NAK received and retries >
‘NAK retries’
or
ACK timeout received and
tries > ‘ENQ retries’ return
error indication
NAK received and retries
< or = ‘NAK retries’
or
ACK timeout received and
tries < or = ‘ENQ retries’
send enquiry
reply timeout received
return error indication
reply packet received
return data
ACK received
Data table