Rockwell Automation 1770, D17706.5.16 Ref Mnl DF1 Protocol Command - Manual (Page 67)

4–13 Using Full-duplex Protocol to Send and Receive Messages
Publication 1770Ć6.5.16 - October 1996
Message Transfer with ReĆTransmission
In this transfer:
•
noise destroys the DLE ACK while also producing invalid
characters at the receiver
•
because of the invalid characters, the receiver changes its last
response variable to a DLE NAK
•
since the DLE ACK was destroyed, the transmitter sends a DLE
ENQ (enquiry), and the receiver returns the DLE NAK
•
the transmitter retransmits the message and the receiver sends an
ACK
•
t he receiver discards the duplicate message (if duplicate message
detection is enabled on your module)
•
reply is successfully returned
xxxx
not full
OK
OK
xxxx
not full
xxxx
DLE STX xxxx DLE ETX BCC/CRC
DL???CK
DLE STX xxxx DLE ETX BCC/CRC
DLE ACK
Source Transmitter Link Receiver Sink
command
reply
(sometime later ...)
DLE NAK
DLE ENQ
(timeout)
???
(duplicate message)
DLE ACK
(timeout)
DLE STX xxxx DLE ETX BCC/CRC
In this transfer, the receiver has no way of knowing that the DLE
ACK it sent to the transmitter was destroyed. If the transmitter’s
ACK timeout is large enough, it is possible that the reply
(i.e., DLE STX xxxx DLE ETX BCC/CRC) comes back before the
transmitter sends the DLE ENQ. Therefore, the reply comes in
before the DLE ACK is received by the transmitter.