Rockwell Automation 1770-KF2 Data Highway or Highway Plus Interface Module User Manual - Manual (Page 64)

Asynchronous Link Protocols
Chapter 4
4-21
Half-duplex protocol is the alternative to full-duplex protocol. You can
select half-duplex protocol by setting communication option Switches 1,
2, and 5 of SW-1. Half-duplex protocol is based on full-duplex but
extends or redefines several features.
Half-duplex protocol is a multi-drop protocol for one master and one or
more slaves. Modems must be used (unless there is only one slave). KF2
modules have slave mode capability only; the master function must be
provided by a user-programmed intelligent device.
Half-duplex protocol provides a lower effective utilization of resources
than full-duplex, but it is much easier to implement. Its use is indicated
by the following:
Multiple stations and a single computer are connected in a multi-drop
configuration using multi-drop modems.
Half-duplex modems are being used.
It is desirable to sacrifice throughput in exchange for ease of
implementation.
Half-duplex protocol requires the following communication
characteristics:
8 Bits per Character
Even Parity
1 Stop Bit
Pass-All Mode (The terminal driver does not translate or respond to
control characters.)
Single Character Buffer (The terminal driver immediately returns each
character to the caller.)
Multi-Drop Topology
The intended environment for half-duplex protocol is a multi-drop 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 dial-up
modems are used, they must also support data-set-ready and
data-terminal-ready; otherwise, DSR should be jumpered to DTR.
There may be from 2 to 256 stations simultaneously connected to a single
link. Each station has a receiver permanently connected to the circuit and
a transmitter that may be enabled or disabled by RTS.
Half-Duplex Protocol