D - application considerations, Introduction, Types of input devices – Rockwell Automation 1756-HSC ControlLogix High Speed Counter Module User Manual
Page 105: Appendix d, Application considerations, Introduction types of input devices
Rockwell Automation Publication 1756-UM007C-EN-P - November 2011
Appendix
D
Application Considerations
Introduction
This appendix provides background for selecting the appropriate input device
for your 1756-HSC module, explains the output circuit, and provides you with
information for selecting the type and length of input cabling.
Types of Input Devices
To turn on an input circuit in the HSC 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 flows through
the photodiode of the opto-isolator and that channel will be off. Its
corresponding input status indicator is off.
All six inputs are electrically identical.
There are two basic classes of driver devices built-in to encoders and other
pulse sources.
• Single-ended
• 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 50/60 Hz AC wiring, use
of shielded cable, twisted pair cables, and so forth. For more information, see
Industrial Automation Wiring and Grounding Guidelines, publication
.