Rockwell Automation 1775-KA PLC-3 Communication Adapter Module User Manual - Manual (Page 128)

Chapter
11
11Ć1
Half-Duplex Protocol
Half–duplex protocol serves as an alternate to full–duplex protocol. Half–
duplex is synonymous with polled–subscriber mode. To select the
half–duplex mode, you select the polled subscriber mode with LIST
(chapter 2).
Half–duplex protocol differs from the full–duplex mode in two ways:
Half–duplex protocol provides for polling of slave stations.
Half–duplex protocol does not allow embedded responses.
Half–duplex protocol is for one master and one or more slaves. You must
use MODEMS for this type of link (unless there is only one slave). The
1775–KA module has slave mode capability only; you must provide the
master function through a computer.
For peer–to–peer communication, half–duplex protocol provides a less
effective use of resources than full–duplex, but it is easier to implement.
You should use half–duplex protocol if:
You are using multidrop baseband MODEMS to connect multiple slave
stations to a single master computer
You are using MODEMS that have only half–duplex capability
You are willing to sacrifice data throughput in exchange for ease of
implementation
One environment for half–duplex protocol is a multidrop link with all
stations interfaced through half–duplex modems. The actual nature of the
link does not matter much, as long as the MODEMS support
request–to–send, clear–to–send, and data–carrier–detect signals. If you
use dial–up MODEMS, they must also support data–set–ready and
data–terminal–ready; otherwise, you should jumper data–set ready to
data–terminal–ready at the 1775–KA module.
You may have from 2 to 256 stations simultaneously connected to a single
multidrop link. Each station must have a receiver connected to the circuit
and a transmitter that can be enabled or disabled by request–to–send.
Half-Duplex Protocol
Multidrop Link