Network Controller 6 Mounting and Wiring Guide
Updated: January 16, 2012
Wiring Details
RS-485 Biasing
16
RS-485 Biasing
The RS-485 port on the controller’s base board has a pair of two-pin jumpers that can be shorted with jumper
blocks to provide “biasing”. As shipped from the factory, these pins are not shorted, thus the RS-485 port is
unbiased.
Note
procedure. In many cases, the
default unbiased RS-485 port is preferred.
Need for RS-485 Bias
Note
A full discussion of communications line termination is beyond the scope of this document.
Biasing sometimes improves RS-485 communications by eliminating “indeterminate” idle states. When you
install two, 2-pin shorting blocks on the controller’s RS-485 bias jumper pins, this adds two onboard 3.3K ohm
resistors into the controller’s RS-485 circuit, as follows:
•
from RS-485 “+” to 5V.
•
from RS-485 “-” to Ground.
Notes
•
In general,
only one device
on an RS-485 trunk should be biased. Otherwise, undue loading of the
circuit may result, with fewer devices supported.
•
RS-485 bias resistors are
different
than “termination resistors”, externally installed at the two
physical ends of a daisy-chained RS-485 trunk, across the “+” and “-” terminals. Termination
resistors are typically 100 or 120 ohm value resistors.
•
Whenever termination resistors are used, RS-485 biasing is typically required.
Adding RS-485 Bias
Note
In many cases
you must furnish the required two 2-pin shorting blocks
. They must be the “shorter
style” jumper blocks. Otherwise, tall blocks interfere when reseating the NPM board. Only units
shipped after mid-2011 may include the jumper shorting blocks, positioned on one pin only of both
2-pin jumpers. See
. Earlier units have only the four bare jumper pins.
To add biasing, you must power off the controller and remove some assemblies (including metal shield, NPM
board) to access the base board jumper pins, located behind the RS-485 port. Then you must install two shorting
blocks on these jumper pins. Then you reassemble the unit by remounting the NPM board, hex standoffs, metal
shield, and shield screws.
Warning
Remove all power from the controller before working on the unit. Observe static discharge
precautions (see
“Static Discharge Precautions,”
).
