Rs485 Many Talkers- Many Listeners, Half Duplex; Chapter 4 Page 46 - Brainboxes CC-310 Hardware Manual

RS422/485 Information
off the main not terminated.
In the above scheme, one RS485 device is talk only, it
transmits data, but it does not receive any. The other RS485 devices
are receive only, they do not transmit any data at all. Figure 4-3
gives the RS485 Gating jumper settings.
Figure 4-3 Half Duplex Settings of RS485 Multiplex
Both jumpers shorted, causing the RXD- & TXD- and the
RXD+ & TXD+ signals to short together at the 9-pin connector.

RS485 Many Talkers- Many Listeners, Half Duplex.

Another popular RS485 layout is for multiple talkers and
multiple listeners. This is shown in Figure 4-4, below. This is also
known as "party line" transmission. It is imperative to have some
method of preventing two devices trying to drive the data lines at
the same time. The normal method is to use the RTS line as a talk
enable. The RTS line should go true immediately prior to the data
transmission and go false immediately after the last byte in the
stream is sent. See Figure 4-3 for jumper settings.
Figure 4-4. RS485 Many Talkers & Listeners. Half Duplex
TXD1+
W
120
TXD1-
-RXD1
+RXD1
Note: BOTH ends of MAIN line terminated in characteristic
impedance, stubs off main line not impedance, since both ends

Chapter 4 Page 46

PCI VELOCITY / OPTO ISO RS422/485
+RXD2 +RXD3
-RXD2 -RXD3
+TXD2 +TXD3
-TXD2 -TXD3
+TXD n
W
120
-TXD n
-RXDn
+RXDn
+RXDn-1
-RXDn-1
+TXDn-1
-TXDn-1