Rockwell Automation 1771-VHSC , D17716.5.74 Very High S - Manual (Page 66)

Application Considerations C–9
Cable Impedance
Generally, you want the cable imedance to match the source and/or
load as closely as possible. Using 150 ohm Belden 9182 (or
equivalent) cable more closely matches the impedance of both
encoder and module input circuits than 78 ohm 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
9pF/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 250KHz is designed to work with
Allen-Bradley Bulletin 845H or similar incremental encoders with a
quadrature specification of 90
o (+22 o ) and a duty cycle specification
of 50% (+10%). Any additional phase or duty cycle changes caused
by the cable will reduce the specified 250KHz specification.
For any application over 100 feet, and/or over 100KHz, use Belden
9182, a high performance twisted-pair cable with 100% foil shield, a
drain wire, moderate 150 ohm inpedance and low capacitance per
unit length.