Advantech PCI-1602UP, User Manual

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PCI/ PCI Express-Communication
User Manual 52
5.2.3 RS-485 Signal Wiring
The RS-485 standard supports half-duplex communication. This means that just two
wires are needed to both transmit and receive data. Handshaking signals (such as
RTS, Request To Send) are normally used to control the direction of the data flow
and to switch the transmission accordingly. In RS-485 mode, the PCI-1601/1602/
1611/1612/1622 cards automatically sense the direction of the data flow and switch
the transmission direction — no handshaking is necessary. This means a user can
build an RS-485 network with just two wires. This RS-485 control is completely trans-
parent to the user. The software written for half duplex RS-232 works without any
modification.
Figure 5.1 RS-485 Wiring Topology
Termination Resistor Setup
You can install termination resistors if necessary for impedance matching. The card
has mounting spaces for termination resistors, but no resistors are installed at the
factory. Depending on your application you may need to solder in a single resistor to
handle the DATA+/DATA- pair (
and a corresponding resistor on the other end of
the connection ). The value of the resistor should equal the characteristic impedance
of the signal wires (approximately 120 Ohms or 300 Ohms).
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