Rockwell Automation 1756-HSC ControlLogix High Speed Counter Module - Manual (Page 110)

Rockwell Automation Publication 1756-UM007C-EN-P - November 2011
110 Application Considerations
If a 24V supply is used, and this driver supplies 15 mA, the output voltage still
would be about 23V (15 mA x 22  = 0.33V, and Vce = .7V).
If the input is applied to the 12-24V terminal, the current to the photodiode is
limited by the series resistance of R3 and R4 (about 1.15 k). A protection
circuit consisting of Q2, R7 and R8 is included. If the current through the
photodiode exceeds about 8 mA, the voltage across R7 and R8 is sufficient to
turn Q2 on, shunting any additional current away from the photodiode. The
voltage drop across Q2 is equal to about 2V (Vphotodiode + Vbe = 2V). The
current demanded by the 1756-HSC input circuit would be about 18 mA (23V
- 2V/1.18 k = 17 mA) which is well within the capability of this driver.
Open Collector
Open collector circuits (the upper circuit on the following circuit) require close
attention so that the input voltage is sufficient to produce the necessary source
current, since it is limited not only by the 1756-HSC input resistors but also
the open collector pull-up.
42628
5V Differential
Line Driver
Input Terminals
14
16
18
13
15
17
R
22

High
Drive
Low
Drive
+12 to 24V
Input
+12 to 24V
Single-ended
Driver
R1
1K
R2
150
C1
R3
1K
R4
150
C2
D2
D3
D5
D6
R7
40.2
R8
40.2
D4
Q2
D1
Q1
R5
40.2
R6
40.2
C3