Rockwell Automation 1770-KF2 Data Highway or Highway Plus Interface Module User Manual - Manual (Page 13)

Chapter
2
2-1
Communication Concepts
This chapter presents some of the concepts of communication with the
KF2 module. It describes the physical communication links to the module
and the various levels of software necessary to make those links work.
A KF2 module connects a computer or programmable controller to an
Allen-Bradley local area network (LAN), either Data Highway or Peer
Communication Link (PCL). In doing so, the module acts as an interface
between two physical communication links:
Data Highway or PCL, Called “Network Link”
RS-232-C or RS-422-A, Called “Asynchronous Link”
A Data Highway network provides peer-to-peer communication between
the KF2 module and other stations on the Data Highway. It uses a
half-duplex (polled) protocol and rotation of link mastership.
A Peer Communication Link provides peer-to-peer communication
between the KF2 module and other stations on the PCL. The PCL uses
token-passing to control access to the link.
The asynchronous link can provide either peer-to-peer communication
through a full-duplex (unpolled) protocol, or master-slave communication
through a half-duplex (polled) protocol.
In addition to a physical link layer, communication on either the network
link or the asynchronous link involves three levels of software:
Application Layer
Network Layer
Data Link Layer
If you are using a computer on an asynchronous link, you must program
all three layers. For A-B programmable controllers, you need program
only the application layer; the Data Highway interface modules (or for the
PCL, the PLC-5 itself) automatically take care of the other two layers.
The rest of this chapter presents some of the concepts behind the physical
communication links and their three software layers. For more details on
General