PCI 1 Port RS422/485
Software Setup
Chapter 3 Page 59
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+
TXD1-
+TXD n
-TXD n
120W
-RXD1
+RXD2
120W
-RXD2
+TXD2
-TXD2
+RXD1
+RXD3
-RXD3
+TXD3
-TXD3
+RXDn-1
-RXDn-1
+TXDn-1
-TXDn-1
-RXDn
+RXDn
Note: BOTH ends of MAIN line terminated in characteristic
impedance, stubs off main line not impedance, since both ends
receive. The twisted pair ends are wired to both RXD+ & TXD+
and RXD- & TXD- at each RS485 device!
