Rockwell-automation 1770-KF2 Data Highway or Highway Plus Interface Mo Instrukcja Użytkownika Pobierz pdf (Strona 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

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Rockwell-automation 1770-KF2 Data Highway or Highway Plus Interface Mo Instrukcja Użytkownika Strona 64