Fluke NetDAQ 2640A, Руководство по обслуживанию

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NetDAQ
Service Manual
2-24
Pulse transformer A1T3 provides electrical isolation between the Ethernet Chip (A1U32)
and the 10BASE-2 transceiver chip (A1U16). Data is transmitted from the Ethernet Chip
(A1U32) to the transceiver chip (A1U16) through pins 1, 2, 15, and 16 of pulse
transformer A1T3. Resistor A1R24 terminates the outputs from the Ethernet Chip
(A1U32). Data is received from the transceiver chip (A1U16) through pins 4, 5, 12, and
13 of pulse transformer A1T3. Resistors A1R42, A1R65, and capacitor A1C33 provide a
termination network for data received through the pulse transformer (A1T4). The
transceiver chip indicates a collision was detected on the Ethernet through pins 7, 8, 9,
and 10 of pulse transformer A1T3. Resistors A1R85, A1R87, and capacitor A1C69
provide a termination network for the collision detected signal received through the
pulse transformer (A1T4).
Digital Inputs and Outputs 2-42.
The following paragraphs describe the digital input/output as follows:
•
Digital Input Threshold
•
Digital Input Buffers
•
Digital and Alarm Output Drivers
•
Totalizer Input
•
External Trigger Circuits
Digital Input Threshold 2-43.
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.
Digital Input Buffers 2-44.
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 (A1U3-2) 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 A1U3-2 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
+1.65V dc, the output of the inverting comparator is low. When the input then drops
below about +1.15V dc, the output of the inverting comparator goes high.