Perle Systems Adaptors - page 137

UltraPort Serial Adaptors User Guide
The EIA-485 Standard Page 137
One Masters /
Many Slaves The EIA-485 Half Duplex mode supports a Master and many Slaves devices. This system 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