Rockwell Automation 1770-KF2 Data Highway or Highway Plus Interface Module User Manual - Manual (Page 44)

Chapter
4
4-1
Asynchronous Link Protocols
This chapter describes the communication protocol used on the
asynchronous link to the KF2 module. If you are connecting a KF2
module to another Allen-Bradley communication interface module (such
as a 1771-KG, 1773-KA, or 1775-KA module), you need not be
concerned with protocol because the modules automatically take care of
it. However, if you are connecting a KF2 module to a computer, then you
must program the computer to understand and to issue the proper protocol
character sequences, as described in Chapters 4 through 6 of this manual.
A link consists of a wire and associated hardware—such as transceivers,
UARTs, and error checkers. A link protocol carries a message error-free
from one end of the link to the other, or it indicates failure with an error
code. Internally it delimits messages, detects and signals errors, retries
after errors, and controls message flow. It requires that the link hardware
send characters from one end of the wire to the other.
The only purpose of a link protocol is to carry a message intact over a
link. It has no concern for the content of the message, the message’s
function in the operation of higher levels in the system, or the ultimate
fate or purpose of the message. Once the message has been reliably
carried from one end of the link to the other, the link protocol’s concern
for that message is ended.
The asynchronous port of the KF2 module can use one of two link
protocols, which are:
Full-Duplex Protocol (for Point-to-Point Communication)
Half-Duplex Protocol (for Master-Slave Communication)
In general, full-duplex protocol gives faster data throughput but is harder
to implement; half-duplex protocol is easier to implement but gives
slower data throughput. Each of these protocols is described
independently in “Full-Duplex Protocol” and “Half-Duplex Protocol”
respectively.
General
Definition of Link Protocol