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RS-422/485 Connections/Operation
34
Chapter 8 Document Number 3PCI-0712m
Figure 17. Setting RTS Control for RS-485
The RS-485 mode is set by configuring the DIP switch setting on the MIport card and by
selecting
RS-485 Mode under RTS Control in the Advanced COM port settings dialog
box.
Note: For more information on COM port settings refer to Chapter 2
and Chapter 5
RS-485 Termination Resistors
In some applications termination resistors must be connected across the
communications line to ensure reliable communications. Termination resistors absorb
signal reflections that can occur when a data signal reaches the end of a transmission
line and encounters an impedance higher than that of the transmission line itself.
Typically termination resistors are not required for communications links that operate at
less that 19.2 kbps. At higher bit rates a termination resistor of 120 ohms may be
required. Termination resistors should be connected across the communications line at
the extreme opposite ends of the network.
Through hole pads are provided on MIport cards to install termination resistors. When
adding termination resistors the value of biasing resistors must be appropriately sized
as well.
RS-485 Network Biasing
Unlike RS-422, where the transmitter holds the TDB(+) line high and the TDA(-) line low
(in the idle, or Mark state) when not transmitting data, in a RS-485 network, the
transmitter tri-states to a high impedance state when not sending data.