Rockwell Automation 1770-KF2 Data Highway or Highway Plus Interface Module User Manual - Manual (Page 74)

Asynchronous Link Protocols
Chapter 4
4-31
summing the BCC, but the data is discarded. If an error is detected, it is
recorded. If any control code other than a DLE ETX BCC is received, the
error flag, the BCC, the message size, and the address (optionally) are all
checked. If any of the tests fail, the message is ignored.
If the message is OK, its header is compared to the last message. If it is
the same, the message is discarded and a DLE ACK is transmitted. If it is
different, the new header is saved and the message is sent to the message
sink. If the message can be stored, a DLE ACK is sent. If the message
sink is full, the message is discarded and not acknowledged.
If while waiting for a message a DLE ENQ is received, the transceiver
accepts the next two characters. The last character is read directly from
the link, since it is a BCC and is not byte stuffed. If the station address
does not match or there is an error, the poll is ignored. If the poll is
accepted, there are three possible situations:
1. The transceiver could still be holding a message that it had
transmitted previously but had not been ACKed. There is a limit on
the number of times each message can be sent. If this limit is
exceeded when the poll is received, the message is returned to the
message source with an error indication, and the transceiver tries to
send the next message from the message source. If the limit is not
exceeded, the response to the poll will be to resend the current
message.
2. If no message is currently being held, the transceiver tries to get one
from the message source. If one is available, the transceiver will
initialize its retry counter and transmit it in response to the poll.
3. If no message is available, the response to a poll will be to transmit a
DLE EOT.
When a message is transmitted after receiving a poll, its format is
identical to a full-duplex message packet. After sending a message, the
transceiver will hold the message until a DLE ACK is received, or the
number of times the message has been polled exceeds the limit.
When a DLE ACK is received, the message currently held is discarded.
When the next poll is received, the next message available from the
message source is sent (or a DLE EOT).