PCMCIA 1 Port RS422/485
Additional 422/485 Operation
Chapter 4 Page 40
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 in less noisy
environments.
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 4-2. 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..
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