Teknor Industrial Computers TEK-CPCI-1000, Technical Reference Manual

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Connecting Devices
7-7
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RS-485 Protocol:
The RS-485 protocol (Half Duplex) also uses differential signals during a communication
session. It differs from the RS-422 mode as it offers the ability to transmit and receive over
the same pair of wires, and allows sharing of the communication line by multiple stations.
This configuration (also known as Party Line) allows only one system to take the control of
the communication line at the time.
The electrical circuit is described below:
3 R X -
4 R X+
120 ohm s
SER IAL PO R T 2
10-PIN HE A D ER
V C C
R XD2
T XD2
VC C
EN A B LE
1
2
7
6
3
4
In RS-485 mode, the RX lines (pins 3 and 4) are used as the transceiver lines, and the RTS
signal is used to control the direction of the RS-485 buffer. Upon power-up or reset, the
transceiver is by default in receiver mode to prevent unwanted perturbation on the line.
Party line operation mode requires termination resistors to be installed at both ends of the
network.
CAUTION
When installing the TEK-CPCI-1000 at one end of the network, W6
and W7 jumper caps must be installed to connect the 120 ohms
termination resistors (See Section 10 - Jumper Settings).