Teknor Industrial Computers PCI-934, Technical Reference Manual

Page: 42 / 124

Installing Peripherals
8-5
RS-485 - Party Line Operation : The RS-485 offers the ability to transmit and
receive over the same pair of wires (RX outputs: pins 3 and 4), and share the same
communication line with multiple stations. To ensure this configuration, only one system
takes the control of the communication line at the time.
3 RX-
4 RX+
120 oh ms
S ER IA L PO R T 2
10 -PIN HE A D ER
VCC
RXD2
TXD2
VCC
EN ABLE
1
2
7
6
3
4
The RS-485 protocol offers some advantages such as increased speed over long
distances, improved reliability over similar RS-232 setups, ability to share the
transmission line, less cabling requirements than RS-422 protocol.
Upon power-up or reset, the transceiver is by default in "receiver mode" to prevent
undesired perturbation on the line.
In RS-485 mode, only both ends of the network must be terminated (120 Ohms
resistors). Termination resistors are available: If the board is installed at one end of the
network, these resistors may be installed using the W2 and W3 jumpers.
Pinout:
When configured for RS-422/485 communication mode, the pin assignation for
the onboard Serial Port 2 (J7) appears as follows:
O n b o a rd
1 0 -p in H ea d e r
1
9
2
10
1
3
5
7
9
D C D
R X (-)
T X (-)
D T R
G N D
Sign al
O nb oard S erial P o rt C onne ctor s et a s R S -485
P in #
2
4
6
8
10
I
O
I
I
-
I
I/O
O
O
-
D S R
R X (+ )
T X (+ )
R I
N .C .
S ignal
P in # I/O I/O