Hdlc Data Link Layer; Chapter Objectives; What Is Hdlc; Layered Approach To Software - Allen-Bradley 1779-KFMR User Manual

Chapter Objectives

What Is HDLC?

Layered Approach to Software

HDLC Data Link Layer

This chapter describes the HDLC frames, the fields within the frames, and
the sequences of transmitting and receiving frames at the data-link layer
for communication along the RS-422/449 link between the host computer
and the 1779-KFM interface.
HDLC stands for High-Level Data-Link Control. HDLC is a
data-link-level communication standard generated by the International
Standards Organization (ISO). The American National Standards
Institute (ANSI) provides an equivalent specification for Advanced Data
Communication Control Procedures (ADCCP) described in ANSI
Publication X3.66-1979.
This chapter and the chapters that follow describe how to write driver
software that enables your computer to communicate with the 1779-KFM
interface. We describe a layered approach to writing driver software for
your computer. You don't have to design your communication software in
this layered fashion, but your software must perform all of 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 a
layered approach.
The Reference Model of Open Systems Interconnection from ISO has
seven layers. However, we do not describe all seven layers of the ISO
model. We describe an application layer that corresponds to the
application layer of the ISO model. We also describe a data-link layer
that corresponds to the data-link layer of the ISO model. However, the
layers we describe between the application and data-link layers
correspond to what is common to all message transmissions in the specific
message set.
This chapter describes HDLC at the data-link layer. The 1779-KFM
module provides several communication protocols from which you can
choose. HDLC at the data-link layer is common to all possible message
sets. Each of the chapters that follow describes a specific protocol and the
protocol layers above the HDLC data-link layer that make that protocol
unique.
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