Honeywell CP-300E Series, Installation Instructions Manual

WEB-300E, WEB-600E/U, CP-300E, CP-600E/U SERIES CONTROLLERS

11

31-00010—01

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: See “Need for RS-485 Bias” before following 

the “Adding RS-485 Bias” procedure. In 

many cases, the default unbiased RS-485 

port is preferred.

Need for RS-485 Bias

NOTE: A full discussion of communication line ter-

mination is beyond the scope of this docu-

ment.

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 “ter-

mination resistors”, externally installed at the 

two physical ends of a daisy-chained RS-485 

trunk, across the “+” and “–” terminals. Termi-

nation resistors are typically 100 or 120 ohm 

value resistors.

—

Whenever termination resistors are used, 

RS-485 biasing is typically required.

Adding RS-485 Bias

To add biasing, you must power off the controller and 

remove a few assemblies (such as metal shield and NPM3E 

or NPM6E processor module) to access the base board 

jumper pins, located behind the RS-485 port. Then you must 

reposition the two shorting blocks on these jumper pins. 

Then you reassemble the unit by remounting the processor 

module, hex standoffs, metal shield, and shield screws.

WARNING

Remove all power from the controller before 

working on the unit.

Observe static discharge precautions. See the 

“Static Discharge Precautions” section.

If the controller is already installed or mounted, it is 

recommended that you remove it first. Then work on a flat, 

stable, well-lit work surface.

Fig. 7. Basic stages of controller disassembly.

Disassembling the controller.

1.

Remove all power from the controller. (See previous 

Warning).

2.

Remove the plastic cover. See “Removing and 

Replacing the Cover” on page 4.

3.

Remove the four Phillips head screws that secure 

the metal shield, and set them aside.

PHILLIPS 

HEAD

SCREWS  

METAL

STANDOFFS 

METAL SHIELD

LIFTED AWAY

COVER REMOVED,

REVEALS METAL SHIELD

NPM3E OR NPM6E

PROCESSOR 

MODULE BOARD

PROCESSOR 

MODULE

LIFTED AWAY

JUMPER BLOCK FOR

RS-485 BIASING

1

2

3

M34995