If required, RS-485 networks are terminated with 120-ohm resistors on each end. The required 120-
ohm resistor must be supplied and installed by the user. When using termination resistors it may be
necessary to increase the line biasing by adding lower value bias resistors in order to generate at least
0.2V across RS-485 line. The suggested value of the bias resistors is 470 ohms. One bias resistor is
installed from the B signal to COM. The second bias resistor is installed from the A signal to +5V.
+5V is available on P8 pin 1 when J14 is installed.
10.2.5
RS-485 Wiring Examples
A typical RS-485 wiring example is shown below. COM1 is shown connected to a
multivariable transmitter such as a Control Microsystems 4000 series MVT. The power for
the transmitter can come from the SCADAPack 350 power input source or can be obtained
from the 24V VLOOP output for possible power savings.
7
6
A
B
P6 – RS-485
1
2
P3 – DC Power In
+
–
+
12Vdc Power
Supply
_
A
B
– PWR
+ PWR
SCADAPack LP
Control Microsystems 4000 series MVT
Figure 18: RS-485 Wiring
SCADAPack 350 Hardware Manual
February 14, 2007
41
