Programming Guide

CHUMP Reference Guide

HDLC protocols typically include the following three types of

stations:

• Primary Station - The host or master station that manages the

data link.

• Secondary Station - A slave station that replies to the Primary

Station.

• Combined Station - A combined master/slave station that

functions as both Primary and Secondary Stations.

HDLC protocols operate in one of the following three modes:

• Normal Response Mode (NRM) - The Primary Station controls

the data link.

• Asynchronous Response Mode (ARM) - The Secondary Station

can talk first.

• Asynchronous Balanced Mode (ABM) - Multiple combined

stations share the data link (multi-master mode).

In HDLC protocols, each piece of data is encapsulated in a frame

by a trailer and a header (see Figure 3). The header contains an

HDLC address and an HDLC control field. The trailer is found at

the end of the frame and contains a Frame Check Sum (FCS). The

FCS detects any errors that may occur during transmission. The

frames are separated by HDLC flag sequences which are

transmitted between each frame and whenever there is no data to

be transmitted.

Figure 3 - HDLC Frame Format