Rockwell Automation 1770-KF2 Data Highway or Highway Plus Interface Module User Manual - Manual (Page 53)

Asynchronous Link Protocols
Chapter 4
4-10
Message Characteristics
Ideally the link protocol should not be at all concerned with the content or
form of the message it is transferring. However, full-duplex protocol
places the following restrictions on the messages that are submitted to it
for transfer:
1. Minimum size of a valid message is 6 bytes, maximum is 250 bytes.
2. Some protocol implementations (point-to-point links to a 1771-KG
module, for example) require that the first byte of a message match
the station address. The receiver will ignore messages that do not
contain the correct address.
3. As part of the duplicate message detection algorithm, the receiver
checks the second, third, fifth, and sixth bytes of each message.
There must be a difference in at least one of these bytes between a
message and the previous one for it to be recognized as distinct
rather than a retransmission of the previous message. If Switch 3 of
Switch Group SW-1 if off, the KF2 module does not implement
duplicate message detection (see Chapter 3, “Asynchronous Link
Features”).
Protocol Definition
Whenever the message source can supply a message and the transmitter is
not busy, it sends a message packet on Path 1. It then starts a timeout and
waits for a response on Path 2.
When a DLE ACK is received, the message has been successfully
transferred. After signaling the message source that the message has been
sent, the transmitter proceeds with the next message.
If a DLE NAK is received, the message will be retransmitted. The
transmitter restarts the timeout and waits again for a response. This can
be repeated several times. There is a user-defined limit to the number of
times a message can be retransmitted. If this limit is exceeded, the
message source will be signaled of the failure; and the transmitter will
proceed with the next message.