Rockwell Automation 1770-KF2 Data Highway or Highway Plus Interface Module User Manual - Manual (Page 54)

Asynchronous Link Protocols
Chapter 4
4-11
If the timeout expires before a response is received, the transmitter sends
a DLE ENQ on Path 1 to request a retransmission of the last response sent
on Path 2. It restarts the timeout and waits for a response.
This too can be repeated several times, and there is a user-defined limit on
the number of timeouts that are allowed per message. If the enquiry limit
is exceeded, the message source will be signaled that the transmission has
failed, and the transmitter proceeds to the next message.
Since there are only two response codes defined, there are no invalid
response codes. If the separator returns an invalid response code, the
transmitter will ignore it. A more precise and detailed description of the
actions of the transmitter appears in Table 4.A in structured English
procedures. Figure 4.7 is a flowchart of the software logic for
implementing the transmitter.
Figure 4.7
Software Logic for Implementing Transmitter
DLE STX Data DLE ETX BCC
Message Packet
T
Retransmit Same Message
Timeout Loop
Received
Yes
T
No
Received
Yes
3*
No
Yes
P
Yes
3*
No
DLE ENQ
No
Timed Out
No
11322
T
P = Recovery Procedure
= Ready to Transmit
* Default Values Used
Legend:
Yes
DLE ACK
?
DLE NAK
?
?
NAKs
Received for this
Message
?
Timeouts
for this
Message
?
by the Module
Next Message