Rockwell Automation 1770-KF2 Data Highway or Highway Plus Interface Module User Manual - Manual (Page 14)

Communication Concepts
Chapter 2
2-2
the application and network layers, refer to Chapters 5 and 6. For more
on the data link layer of the asynchronous link, refer to Chapter 4 and
Appendix D.
The physical link layer is a set of cables and interface modules that work
together to provide a channel for communication between the various
points, called stations, on the physical link. A station consists of an
intelligent programmable device (e.g., PC or computer) and the module or
modules that interface it with the physical link.
In this way, the KF2 module allows stations on one link to communicate
with stations on the other link. Since these two physical links have
different communication protocols, the KF2 module serves mainly as a
protocol translator.
Data Highway
The Data Highway and the PCL are both local area networks (LANS) that
allow peer-to-peer communication among up to 64 stations. Figure 2.1
illustrates a Data Highway network.
The Data Highway link consists of a trunkline that can be up to
10,000 feet long and droplines that can be up to 100 feet each. Each
station is at the end of a dropline.
A Data Highway link implements peer-to-peer communication through a
modified token-passing scheme called the floating master. With this
arrangement, each station has equal access to become the master. The
stations bid for temporary mastership based on their need to send
information.
Unlike a master/slave relationship, a floating master relationship does not
require the current master to poll each station to grant permission to
transmit. Therefore, it provides a more efficient network because there is
less overhead per transaction.
Physical Link Layer