Rockwell Automation 1771-VHSC , D17716.5.74 Very High S - Manual (Page 58)

Appendix
C
Application Considerations
This appendix will provide you with background for selecting the
appropriate input device for your 1771-VHSC module, explain the
output circuit, and provide you with information for selecting the
type and length of input cabling.
To turn on an input circuit in the VHSC module, you must source
current through the input resistors sufficient to turn on the
opto-isolator in the circuit.
If no connection is made to a pair of input terminals, no current will
flow through the photodiode of the opto-isolator and that channel
will be off. Its corresponding input status indicator will be off.
All 12 inputs are electrically identical.
There are 2 basic classes of driver devices built-in to encoders and
other pulse sources: single-ended and differential. A single-ended
driver output consists of a signal and a ground reference. A
differential driver consists of a pair of totem-pole outputs driven out
of phase. One terminal actively sources current while the other sinks,
and there is no direct connection to ground.
Differential line drivers provide reliable, high speed communication
over long wires. Most differential line drivers are powered by 5V,
and are more immune to noise than single-ended drivers at any
operating voltage.
Any installation must follow customary good wiring practices:
separate conduit for low voltage dc control wiring and any 50/60Hz
ac wiring, use of shielded cable, twisted pair cables, etc. Refer to
publication 1770-4.1, “Programmable Controller Wiring and
Grounding Guidelines” for more information.
The following examples will help you in determining the best input
type for your particular application. These examples include:
5V differential line driver
single-ended driver
open collector circuit
electromechanical limit switch
Appendix Objectives
Types of Input Devices
Examples for Selecting
Input Devices