Common rs–485 topologies, Straight–line topology – Rockwell Automation 1400-SP PowerMonitor Software User Manual

Page 72

Appendix B
Communications Application Note

B–4

Figure B–1
RS–485 Straight Line Topology

ООО

ОООО

RS–485 Terminals

–

SHLD

C01 RS–485 Terminals

–

SHLD

–

SHLD

Device #1

Device #2

Device #3

C01 RS–485 Terminals

RS–485 Cable:

22 gauge shielded

twisted pair

IBM PC

RS–232C/RS–485

Converter

RS–232C

–

SHLD

There are many topologies that can be used to connect Powermonitors to a
computer using an RS–485 communications path. The two most common
are the straight–line and loop topologies.

Straight–Line Topology

For straight–line topology, the maximum length of the RS–485 cable is 4000
feet. A maximum of 32 devices can be connected at any location along the
length of the communications cable (see Figure B–1).

The computer used to display the data can be located anywhere along the
4000 foot length of the cable.

is a 1/4 watt termination resistor having an impedance equal to the

characteristic impedance of the RS–485 cable. Typical values for R

range

from 130 ohms to 180 ohms and depend on the gauge of the wire used, the
diameter of the insulation and the insulation material. The characteristic
impedance of a two–wire twisted pair cable is normally included with the
electrical specification for the cable.

Common RS–485 Topologies