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at either end in its characteristic impedance, which for the Belden 9729 cable is 120
Ohms. Unscreened Belden type 8795 may also be used in less noisy environments.
There are several schemes for connecting RS485 devices depending on the
characteristics of the system. In many cases there will be only one device, which can
transmit, data and all the others simply listen to it. This scheme is used for theatrical
lighting intensity control in the DMX512 standard. This is shown below. For the talker
the RS485 TXD GATE jumper should remain in the factory set position, i.e. transmitter
is always enabled. There is NO multiplexing of the TXD and RXD lines. Data is only
flowing one way, from PC outwards, and is thus a Half-Duplex configuration, only one
twisted pair cable is needed.
Note: The Receiver end of MAIN line terminated in characteristic impedance by
ONBOARD resistor networks, stubs 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.
Both jumpers shorted, causing the RXD- & TXD- and the RXD+ & TXD+ signals to
short together at the 9-pin connector.
Another popular RS485 layout is for multiple talkers and multiple listeners. This is
shown 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.
TXD1+
120W
TXD1-
-RXD1
+RXD1
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!
+RXD2
+RXD3
-RXD2
-RXD3
+TXD2
+TXD3
-TXD2
-TXD3
+TXD n
120W
-TXD n
-RXDn
+RXDn
+RXDn-1
-RXDn-1
+TXDn-1
-TXDn-1
•
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