Rs485 Cable; Rs485 One Talker - Many Listeners, Half Duplex - Brainboxes CC-357 User Manual

PCI VelocityRS232/422/485 Additional RS422/485 Operation

RS485 Cable.

For best noise immunity use twisted pair cables to make the RS485
connection. In Half Duplex wiring only 1 twisted cable pair is
needed. Two twisted pair cables are needed for Full Duplex
communications.
Use screened twisted pair Belden cable 9729 and 9829, UL type
2493 and 2919 or IBM Part No 4716748 cable to make the RS485
connection. Terminate the twisted pair cable at either end in its
characteristic impedance, which for the Belden 9729 cable is 120
Ohms. Unscreened Belden type 8795 may also be used.

RS485 One Talker - Many Listeners, Half Duplex.

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 in Figure
4-2, 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.
Figure 5-2. RS485 1 Talker Many Listeners.
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. Figure 5-3 gives the RS485 Gating jumper settings.
Chapter 5 Page 75