Rockwell Automation 1756-HSC ControlLogix High Speed Counter Module - Manual (Page 113)

Rockwell Automation Publication 1756-UM007C-EN-P - November 2011
Application Considerations 113
Application Considerations
A successful installation depends on the type of input driver, input cable
length, input cable impedance, input cable capacitance, and frequency of the
input.
The following provides information on these installation factors for the
1756-HSC module.
Input Cable Length
Maximum input cable length depends on the type of output driver in your
encoder, the kind of cable used, and maximum frequency at which you will be
running. With a differential line driver, 250 ft or less of high quality, low
capacitance cable with an effective shield, and an operating frequency of
250 kHz or less will likely result in a successfully installation.
If you use an open collector, or other single-ended driver, at distances of 250 ft
and frequencies of 250 kHz, your chances of success are low. Refer to the table
for suggested desirable driver types.
Totem-pole Output Devices
Standard TTL totem-pole output devices, such as 7404 and 74LS04, are
usually rated to source 400 µA at 2.4V in the high logic state. This is not
enough current to turn on a 1756-HSC input circuit. If your present encoder
has this kind of electrical output rating, you cannot use it with the 1756-HSC
module.
Most encoder manufacturers, including Allen-Bradley, offer several output
options for a given encoder model. When available, choose the high current
5V differential line driver.
Desirable Adequate Undesirable
5V Line Drivers, such as:
DM8830
DM88C30
75ALS192 or equivalent
Balanced Single-Ended: any
AC or ACT family part
or
Discrete, balanced circuit
or
Open-Collector:suitable for
frequencies of <50 kHz
Standard TTL or
LSTTL Gates