Rockwell Automation 1775-KA PLC-3 Communication Adapter Module User Manual - Manual (Page 114)

Full-Duplex Protocol
Chapter 10
10Ć10
timeout, and waits for a response on path 2. You can use the diagnostic
set timeout command to set this timeout period for the 1775–KA module.
The default setting is 3 seconds.
When transmitter A gets a DLE ACK, the message transfer is complete.
After signaling the message source that the message has been sent
successfully, transmitter A proceeds with the next message.
If transmitter A gets a DLE NAK, it retransmits the same message. The
transmitter restarts the timeout and waits again for a response. By using
the diagnostic set NAKs command, you can specify how many times the
1775–KA module will attempt to retransmit a given message. The default
setting is 3. Once the number of retransmissions exceeds this limit, the
transmitter should notify the message source that the transmission has
failed. The transmitter can then proceed with the next message.
If the timeout expires before transmitter A gets a response, it sends a DLE
ENQ on path 1 to request a retransmission of the last response sent on
path 2. Transmitter A restarts the timeout and waits for a response. By
using the diagnostic set ENQs command, you can specify how many
timeout periods the 1775– KA module will allow per message it transmits.
The default setting is 10. If this ENQ limit is exceeded, the transmitter
notifies the message source that the transmission has failed. The
transmitter can then proceed with the next message.
DLE ACK and DLE NAK are the only response codes defined. If the
receiver gets an invalid response code, it ignores it.
Note that the transmitter must encode a text value of 10 hex as two
consecutive (indivisible) bytes, each of value 10 hex. This is necessary to
distinguish the text value of 10 hex from the DLE control code of 10 hex.
This technique is known as DLE stuffing. The receiver must be able to
reverse this process and extract the original text value of 10 hex.
Figure 10.7 is a flowchart which gives a simplified view of an example of
software logic for implementing the transmitter. Table 6.A gives a
detailed description of an example of software logic for implementing the
transmitter in structured English procedures. In appendix D are
flowcharts which give a detailed view of an example of software logic for
implementing the transmitter.
Figure 10.7
Transmitter for Full-Duplex Protocol