Fluke 2620A, Manual

Page: 101 / 310

Theory of Operation (2635A)
Detailed Circuit Description
2A
2A-19
2A-42. Digital I/O
The following paragraphs describe the Digital Input Threshold, Digital Input Buffers,
Digital and Alarm Output Drivers, Totalizer Input, and External Trigger Input circuits.
2A-43. Digital Input Threshold
The Digital Input Threshold circuit sets the input threshold level for the Digital Input
Buffers and the Totalizer Input. A fixed value voltage divider (A1R36, A1R37) and a
unity gain buffer amplifier (A1U8) are the main components in this circuit. The voltage
from the divider (approxi1.4V dc) is then buffered by A1U8, which sets the
input threshold. Capacitor A1C29 filters the divider voltage at the input of A1U8.
2A-44. Digital Input Buffers
Since the eight Digital Input Buffers are identical in design, only components used for
Digital Input 0 are referenced in this description. If the Digital Output Driver (A1U17-
12) is off, the input to the Digital Input Buffer is determined by the voltage level at
A1J5-10. If the Digital Output Driver is on, the input of the Digital Input Buffer is the
voltage at the output of the Digital Output Driver.
The Digital Input Threshold circuit and resistor network A1Z1 determine the input
threshold voltage and hysteresis for inverting comparator A1U3. The inverting input of
the comparator (A1U3-13) is protected by a series resistor (A1Z3) and diode A1CR14. A
negative input clamp circuit (A1Q9, A1Z2, and A1CR17) sets a clamp voltage of
approxi0.7V dc for the protection diodes of all Digital Input Buffers. A negative
input voltage at A1J5-10 causes A1CR14 to conduct current, clamping the comparator
input A1U3-13 at approximately 0V dc.
The input threshold of +1.4V dc and a hysteresis of +0.5V dc are used for all Digital
Input Buffers. When the input of the Digital Input Buffer is greater than approximately
+1.25V dc, the output of the inverting comparator is low. When the input then drops
below about +0.75V dc, the output of the inverting comparator goes high.
2A-45. Digital and Alarm Output Drivers
Since the 12 Digital Output and Alarm Output Drivers are identical in design, the
following example description references only the components that are used for Alarm
Output Driver 0.
The Microprocessor controls the state of Alarm Output Driver 0 by writing to the Alarm
Output register in the FPGA (A1U25) to set the level of output A1U25-63. When
A1U25-63 is set high, the output of the open-collector Darlington driver (A1U17-16)
sinks current through current-limiting resistor A1R62. When A1U25-63 is set low, the
driver output turns off and is pulled up by A1Z2 and/or the voltage of the external device
that the output is driving. If the driver output is driving an external inductive load, the
internal flyback diode (A1U17-9) conducts the energy into MOV A1RV1 to keep the
driver output from being damaged by excessive voltage. Capacitor A1C58 ensures that
the instrument meets electromagnetic interference (EMI) and electromagnetic
compatibility (EMC) performance requirements.
2A-46. Totalizer Input
The Totalizer Input circuit consists of Input Protection, a Digital Input Buffer circuit,
and a Totalizer Debouncing circuit. The Digital Input Buffer for the totalizer is protected
from electrostatic discharge (ESD) damage by A1R49 and A1C43. Refer to the detailed
description of the Digital Input Buffer circuit for more information.