Perle Systems Adaptors manual EIA-485 Half Duplex Diagram #1

One Masters / The EIA-485 Half Duplex mode supports a Master and many Slaves devices. This system

Many Slaves has only one master device which can transmit to multiple slave devices. In many cases there will be only one maser device, which can transmit data and the other simply receives it.There is no multiplexing of the TXD and RXD lines. The data is flowing only in one direction and needs only one pair of wires.

EIA-485 Half

Duplex Diagram #1

Another popular layout is for multiple masters and slaves in EIA-485 half duplex mode. This is also known as a party line transmission. It is imperative to have some method of preventing two devices from driving the line at the same time. The normal method is to use the RTS as a talk enable control. The RTS line will go true immediately prior to transmission and go false immediately after the last byte in the data stream is sent.

Note: Pin numbers provided in above diagrams are for UltraPort SI DB9 connectors

UltraPort Serial Adaptors User Guide

The EIA-485 Standard

Page 137