Advantech PCI-1612 User Manual download pdf (Page 76)

PCI/ PCI Express-Communication User Manual 66

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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Advantech PCI-1612 User Manual Page 76