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Typically, each end of the RS-422/485 bus must have line-terminating
resistors. A 100-ohm resistor is provided across each RS-422/485 input, and
a 1K-ohm pull-up/pull-down combination biases the receiver inputs. If
more than two PC
bus, the 100-ohm resistor must be removed.
Several of the card's options are user-selectable. For example, you can
switch-select which address it uses. Also, the card's driver can be either
permanently enabled (tied high) or enabled depending on the state of the
UART's Request to Send (RTS) signal (the factory default). If you are using
the card in a multipoint or two-wire half-duplex RS-485 environment, the
driver must be set to track RTS and the PC's software must control the
UART's RTS signal. For more information on the card's selectable options,
see the next chapter.
422/485 Serial Interfaces are connected together in a
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CHAPTER 2: Introduction
9
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