Fluke NetDAQ 2640A, Руководство по обслуживанию

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Theory of Operation
Detailed Circuit Description
2
2-43
Communication with the main processor is done using the IGDR line to receive and the
IGDS line to send serial data. On the A/D side, these signals are called RECV DATA
and XMIT DATA (pins A3U5-53 and A3U5-54 respectively). The RESET* signal is
asserted on power-up for reset and during operation when a break signal is received from
A1U4.
The A/D microprocessor guard crossing is bidirectional. When the user finishes defining
the channels and intervals and starts scanning, the A1 Main PCA downloads all the
channel information to the A3 A/D Converter PCA. The A1 Main PCA uses the guard
crossing to advise the A3 A/D Converter PCA when to start scans, and then return the
readings to the A1 Main PCA. The arrangement keeps the guard-crossing traffic to a
minimum when scanning is taking place allowing peak performance during short scan
intervals.
Open Thermocouple Detect Circuitry 2-74.
The open thermocouple detect circuitry uses devices A3U23 and A3U32. Before every
thermocouple measurement the open T/C check is done by sending a small ac-coupled
signal to the thermocouple input. The A/D Microcomputer (A3U5) initiates the open T/C
test by asserting OTC_EN and turning ON A3Q20. A 19.2 kHz square wave is sent out the
OTCCLK line through A3Q20 and A3C82 to the thermocouple.
The resulting waveform is detected by A3U32 pin 3 and a proportional level is stored on
A3C79. If the level is above a threshold level of about 2.7V (Vth) the resistance at the input
is too large (greater than 4 kohm to 10 kohm) and open T/C check is asserted by A3U32
pin 7. After a short delay the A/D Microcontroller reads the signal and determines if the
thermocouple should be reported to the main processor as open.
A4 Analog Input PCA Circuit Description 2-75.
The Input Connector assembly, which plugs into the A/D Converter PCA from the rear
of the instrument, provides 20 pairs of channel terminals for connecting measurement
sensors. This assembly also provides the reference junction temperature sensor circuitry
used when making thermocouple measurements.
Circuit connections between the Input Connector and A/D Converter PCAs are made via
connectors A4P1 and A4P2. Input channel and earth ground connections are made via
A4P1, while temperature sensor connections are made through A4P2.
Input connections to channels 1 through 20 are made through terminal blocks TB1 and
TB2. Channel 1 and 11 HI and LO terminals incorporate larger creepage and clearance
distances and each have a metal oxide varistor (MOV) to earth ground to clamp voltage
transients. MOVs A4RV1 through A4RV4 limit transient impulses to the more
reasonable level of approximately 1800V peak instead of the 2500V peak that can be
expected on 240V ac, IEC 664 Installation Category II, ac mains. In this way, higher
voltage ratings can be applied to channels 1 and 11 than can be applied to the other rear
channels.
Strain relief for the user’s sensor wiring is provided both by the Connector PCA housing
and the two round pin headers. Each pin of the strain relief headers is electrically
isolated from all other pins and circuitry.