Rockwell Automation 1775-KA PLC-3 Communication Adapter Module User Manual - Manual (Page 100)

Chapter
9
9Ć1
Computer to PC Communication
This chapter and the chapters that follow (10,11, and 12) described how to
write a software driver that enables your computer to communicate
through the RS–232–C port of the PLC–3 Communication Adapter
Module. Therefore, you do not need to read these chapters if you are only
using PC’s. The interface modules contain software drivers for PC to PC
communication.
In this chapter and the chapters that follow (10,11, and 12) we describe a
layered approach to writing a software driver for your computer.
According to the standard for network architecture developed by the
International Standards Organization (ISO), communication networks
should be divided into layers. Each layer performs specific functions. By
separating the communication network into independent layers, it is easier
to make changes to one of the network’s functions without having to
redesign the entire network. Ideally, the layers of a network should be as
independent of one another and interact with one another in the same way
as the organs of the human body. Because the organs of the human body
are independent of one another, it’s possible for a surgeon to operate on
the lungs or heart without losing the life of the patient. Yet at the same
time the organs of the body interact when we run or walk or type on a
word processor.
You should use a layered approach to developing communication software
for your computer. You don’t have to design your communication
software in this layered fashion, but your software must perform all the
functions described for the layers in this manual. In most cases, it will be
easier for you to implement and debug the communication software if you
follow this layered approach.
The Data Highway uses these four layers of the ISO model for
communication between stations:
application layer – provides the Data Highway commands that you use
to transfer data and manage the network.
network layer – determines how you address a Data Highway
command. It also provides less visible functions, such as controlling
the flow of information, establishing a path between stations, and
routing messages from your station to another station.
Introduction to Layered
Communication