Rockwell Automation 1775-KA PLC-3 Communication Adapter Module User Manual - Manual (Page 104)

Computer to PC Communication
Chapter 9
9Ć5
We use the term protocol to describe the relationship between two similar
layers at two different stations. The protocol could, for example, be the
relationship between the data link layer at station A and the data link layer
at station B.
When you write a software driver for your computer, you may implement
either a data link layer that uses a half–duplex protocol or a data link layer
that uses a full duplex protocol. You select the half–duplex or full–duplex
protocol with the LIST function (section titled Modem Port, chapter 2).
In general, the full–duplex protocol provides higher data throughput, but
you can use it only for communication between two peer stations.
Half–duplex protocol is for one master and one or more slaves.
Half–duplex protocol provides lower data throughput but is easier to
implement than the full–duplex protocol. You should use half–duplex
protocol if:
You are using multidrop baseband MODEMS to connect multiple slave
stations to a single master computer. (You must use MODEMS for this
type of link unless there is only one slave).
You are using MODEMS that have only half–duplex capability
You are willing to sacrifice data throughput in exchange for ease of
implementation
Half–duplex protocol does not allow embedded responses. The 1775–KA
module has slave mode capability only; you must provide the master
function from your computer.
Full-Duplex vs Half-Duplex
Protocol for the Data Link Layer