Rockwell Automation 1756-HSC ControlLogix High Speed Counter Module - Manual (Page 114)

Rockwell Automation Publication 1756-UM007C-EN-P - November 2011
114 Application Considerations
Cable Impedance
Generally, you want the cable impedance to match the source and/or load as
closely as possible. Using 150  Belden 9182 (or equivalent) cable more
closely matches the impedance of both encoder and module input circuits than
78  cable, such as Belden 9463. A closer impedance match minimizes
reflections at high frequencies.
Termination of one, or both ends, of the cable with a fixed resistor whose
value is equal to the cable impedance will not necessarily improve ‘reception’ at
the end of the cable. It will, however, increase the DC load seen by the cable
driver.
Cable Capacitance
Use cable with a low capacitance as measured per unit length. High
capacitance rounds off incoming square wave edges and takes driver current to
charge and discharge. Increasing cable length causes a linear increase in
capacitance, which reduces the maximum usable frequency. This is especially
true for open collector drivers with resistive pull-ups. For example, Belden
9182 is rated at a very low 9 pF/foot.
Cable Length and Frequency
When cable length or frequency goes up, your selection of cable becomes even
more critical. Long cables can result in changes in duty cycle, rise and fall
times, and phase relationships. The phase relationship between channels A
and B in Encoder X1 and X4 mode is critical.
The maximum encoder input of 250 kHz is designed to work with
Allen-Bradley Bulletin 845H or similar incremental encoders with a quadrature
specification of 90°
(22°) and a duty cycle specification of 50% (10%). Any
additional phase or duty cycle changes caused by the cable will reduce the
specified 250 kHz specification.
For any application over 100 ft, and/or over 100 kHz, use Belden 9182, a
high-performance, twisted-pair cable with 100% foil shield, a drain wire,
moderate 150  impedance and low capacitance per unit length.