Rockwell Automation 1756-HSC ControlLogix High Speed Counter Module - Manual (Page 108)

Rockwell Automation Publication 1756-UM007C-EN-P - November 2011
108 Application Considerations
Thus, V
drop
never exceeds about 2.0V regardless of the applied voltage. In
addition, it is never less than 1.5V if the minimum of 4 mA is flowing.
Although there are some minor temperature effects on the photodiode drop,
you can expect the value V
drop
to be relatively linear from about 1.6V to 2.0V
as the current increases from 4-8 mA.
Look at the following 5V differential line driver example to see why this is
important.
5V Differential Line Driver Example
You want to use a 5V differential line driver in your encoder when you have a
long cable run, and/or high input frequency or narrow input pulses (input
duty cycle < 50%). The top circuit (
page 106 ) shows a typical 5V differential
line driver. The encoder output is connected to the field-wiring arm terminal
16 and is sourcing current and the encoder output to terminal 18 is sinking
current.
To be sure that your device drives the 1756-HSC, you must know the electrical
characteristics of the output driver component used in your signal source
device. The output voltage differential V
diff
= (V
oh
- V
ol
) is critical, because
this is the drive voltage across the 1756-HSC input terminals 16 and 18, and
the photodiode current is a function of Vdiff - Vdrop.
The manufacturer of your shaft encoder or other pulse-producing device can
provide information on the specific output device used.
Many popular differential line drivers, such as the 75114, 75ALS192, and the
DM8830 have similar characteristics and can source or sink up to 40 mA.
In general, the output voltage V
oh
is higher both as the supply voltage and the
ambient temperature increase. For example, vendor data for the 75114 shows
V
oh
is about 3.35V at V
cc
= 5 V, I
oh
= 10 mA and 25 °C (77 °F). V
ol
is about
0.075V under the same conditions. This means V
differential
= V
oh
- V
ol
=
3.27V if the part is sourcing 10 mA. Looking at the curves, if the part were
sourcing 5 mA you would see V
diff
= 3.425 - 0.05 = 3.37V.
IMPORTANT
Neither output of the differential line driver can be connected to
ground. Damage could occur to your driving device.
IMPORTANT
Any signal source that uses a standard TTL output device driver rated
to source 400 µA or less in the high logic state is not compatible with
the 1756-HSC module.