Rockwell Automation 1770-KF2 Data Highway or Highway Plus Interface Module User Manual - Manual (Page 56)

Asynchronous Link Protocols
Chapter 4
4-13
Receiver Actions
Since the receiver gets “dirty” input from the physical world, it is more
complex and must be capable of responding to many adverse situations.
Some of the things that can conceivably happen are listed here:
1. The message sink can be full, leaving the receiver with nowhere to
put a message.
2. A message can contain a parity error.
3. The BCC can be invalid.
4. The DLE STX or DLE ETX BCC may be missing.
5. The message can be too long or too short.
6. A spurious control or data code can occur outside a message.
7. A spurious control code can occur inside a message.
8. The DLE ACK response can be lost, causing the transmitter to send a
duplicate copy of a message that has already passed to the message
sink.
A record of the last response sent on Path 2 is kept. The value of this
response is either ACK or NAK. It is initialized to NAK. When a DLE
ENQ is received, the receiver sends the value of the last response from
this variable.
A record is kept of several message header bytes. If a message has the
same header as the previous message, the message is ACKed but
discarded.
The receiver ignores all input from Path 1 until a DLE STX or a DLE
ENQ is received. If anything other than a DLE STX or DLE ENQ is
received on Path 1, the receiver sets the last response variable to a NAK.
If an ENQ is received, the last response is sent on Path 2; and the receiver
continues waiting for input.