Fluke 2680A, Service Manual, Page: 61 / 254

Theory of Operation
Digital Input/Output PCA
2
2-13
Digital Input Buffers
Since the 20 Digital Input Buffers are identical in design, only components used for
Digital Input 0 are referenced in this description. If the Digital Output Driver (U13 Pin
13) is “off”, the input to the Digital Input Buffer is determined by the voltage level at P6
Pin A1. 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 Z2 determine the input threshold
voltage and hysteresis for inverting comparator U8. The inverting input of the comparator
(U8 Pin 13) is protected by a series resistor (Z6) and diode CR9. A negative input clamp
circuit (Q1, R7, and CR1) sets a clamp voltage of approxi0.7 V dc for the
protection diodes of all Digital Input Buffers. A negative input voltage at P6 Pin A1
causes CR9 to conduct current, clamping the comparator input U8 Pin 13 at
approximately 0 V dc. Maximum input voltage is limited to +30 V dc. R8 provides input
protection should the input 30 V dc. The maximum current with the output
asserted (low) is 500 mA.
The input threshold of +1.4 V dc and a hysteresis of +0.5 V dc are used for all Digital
Input Buffers. When the input of the Digital Input Buffer is greater than approximately
+1.65 V dc, the output of the inverting comparator is low. When the input then drops
below about +1.15 V dc, the output of the inverting comparator goes high.
Digital Output Drivers
Since the 20 Digital Output Drivers are identical in design, the following example
description references only the components that are used for the Digital Output (DO<0>).
The microprocessor controls the state of the Digital Output Driver DO<0> by setting the
level of the output U4 Pin 59. When U4 Pin 59 is set high, the output of the open-
collector Darlington driver (U13 Pin 13) sinks current through current-limiting resistor
R80. When U4 Pin 59 is set low, the driver output turns off and is pulled up by Z4 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 (U13 Pin 9) conducts the energy
into MOV RV1 to keep the driver output from being damaged by excessive voltage.
Capacitor C127 ensures that the instrument meets electromagnetic interference (EMI) and
electromagnetic compatibility (EMC) performance requirements.
Relay Output Drivers
Since the 8 Relay Output Drivers are identical in design , the following example
description references only the components that are used for the operation of K1.
The output relays are latching relays and therefor must be set/reset individually. The
Microprocessor controls the state of the Digital Output Driver DO<20> by setting the
level of the output U4 Pin 67. When U4 Pin 67 is set high, the output of the open-
collector Darlington driver (U10 Pin 10) sinks current through the K1 Set relay coil; the
internal flyback diode (U10 Pin 9) conducts the energy into Vcc to keep the driver output
from being damaged by excessive voltage when the relay coil is de-energized.
When U4 Pin 68 is set high, the output of the open-collector Darlington driver (U10
Pin 11) sinks current through the K1 Reset relay coil; the internal flyback diode (U10 Pin
9) conducts the energy into Vcc to keep the driver output from being damaged by
excessive voltage when the relay coil is de-energized.
K1 is a latching double pole, double throw relay. The relay is configured in a parallel-
ganged configuration; when the relay is closed, P5 Pin B30 will be connected to P5 Pin
C29 and P5 Pin B32 will be connected to P5 Pin C31. Fuses F1 and F5 (2A) protect the
relay contacts from excessive current.