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Chapter

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

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Plus RS-232 Manuel d'utilisateur Page 44