Rockwell Automation 1770-KF2 Data Highway or Highway Plus Interface Module User Manual - Manual (Page 18)

Communication Concepts
Chapter 2
2-6
Computer Programming
The communication protocol for the PCL is transparent to a computer on
the network. However, for a computer to send or receive messages
through the PCL, it must be programmed to communicate with its KF2
module over an asynchronous link. Chapters 4, 5, and 6 describe the
protocol that you must program your computer to use on this link.
Configuration Selection
Figure 2.1 and Figure 2.2 illustrate configurations in which PC stations
can communicate with each other and with computers through network
ports and asynchronous ports on the station interface modules. Each
configuration is useful, depending on your application.
If you want to provide a peer-to-peer communication among many PCs
and/or a computer, use a Data Highway network as shown in Figure 2.1.
For communication among a small cluster of PCs and a computer, use a
PCL as shown in Figure 1.4. For distances longer than the networks
provide, you can use an auxiliary longline asynchronous (RS-232-C or
RS-422-A) link or a modem link.
A Data Highway link has a communication rate of 57,600 bits per second
and a half-duplex (peer-to-peer, polled) protocol. A KF2 asynchronous
link is selectable RS-232-C/RS-422-A and has a selectable
communication rate up to 9,600 bits per second. It uses a selectable
protocol of half-duplex (master-slave polled) or full-duplex (peer-to-peer,
unpolled).
A master-slave communication protocol can be selected for any link to a
computer. A peer-to-peer communication protocol can be selected only
for a point-to-point link or a broadband modem multi-drop link to a
computer.
Even with only two stations, you may want a Data Highway or a Peer
Communication Link. Either network provides the flexibility of easy
reconfiguration or expansion if you want to be able to add more stations
later, and it also provides more error checking than an asynchronous link.