Plus RS-232 Manuel d'utilisateur télécharger pdf (Page 228)

Appendix

E-1

Data Link Layer

The data link layer controls the flow of communication on the physical

link by acquiring and releasing access to the communication channel for

each station. This layer differs for each type of physical link.

Chapter 4 explains how to program the data link layer for an

asynchronous link. The rest of this section describes the data link layer of

the Data Highway link.

Note that you do not have to program the data link layer for the Data

Highway; the communication interface modules automatically take care

of it. The description of it presented here is solely for information

purposes.

The protocol that is used between stations on the Data Highway link is a

modified low-level implementation of HDLC that features bit stuffing,

flag definition, and generation of the cyclic redundancy check (CRC).

Central to the interaction of Data Highway modules is the concept of the

floating master. With this arrangement, no single station is permanent

master controlling the Data Highway communication link at all times.

Instead, each station bids for mastership, based on its need to send

command or reply messages. This arrangement has two major features:

Multiple Masters

Peer-to-Peer Communication

One advantage of floating master operation is that no single station

disables communication on the Data Highway as long as other stations

continue to operate. This means that even with disconnection or faulted

operation of a module or a processor, communication between other

operating station interface modules continues. This minimizes the need

for backup in some applications.

Data Link Layer Operation on

Data Highway

Floating Master

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Plus RS-232 Manuel d'utilisateur Page 228