User manual

PCL-COMMUNICATION User Manual 70
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. Hand-
shaking 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 communication card automatically
senses the direction of the data flow and switches the transmission direc-
tion — no handshaking is necessary. This means a user can build an RS-
485 network with just two wires. This RS-485 control is completely
transparent to the user. The software written for half duplex RS-232
works without the need for any modification.
Figure 5.15: RS-485 Wiring Topology
5.3 Terminator Resistor Setup
You can install termination resistors if necessary for impedance match-
ing. The card has jumpers 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 cor-
responding resistor on the other end of the connection). The value of the
resistor should equal the characteristic impedance of the signal wires
(approximately 120 or 300 Ohms). The following table shows every
card’s terminator resistor.
Item Port 1 Port 2 Port 3 Port 4
PCL-740 TR1
PCL-743S/B
PCL-745S/B
TR1 TR3
PCL-746+ TR1 TR2 TR3 TR4
PCL-846A/B TR1 TR5 TR9 TR13