Fluke 2680A Service Manual Download

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268XA

Service Manual

2-28

Communication with the Controller/System Power PCA is done using the CordxRx line
to receive and the CordxTx 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 bi-directional. When the user finishes defining
the channels and intervals and starts scanning, the Controller/System Power PCA
downloads all the channel information to the A3 A/D Converter PCA. The
Controller/System Power PCA uses the guard crossing to advise the A3 A/D Converter
PCA when to start scans, and then return the readings to the Controller/System Power
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

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 2.7 V (Vth) the resistance at the
input is too large (greater than 4 k

Ω

to 10 k

Ω

) 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 Controller/System Power PCA as open.

A4 Analog Input PCA Circuit Description

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 1800 V peak instead of the 2500 V peak that can be
expected on 240 V 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.