Fluke 2680A, Руководство по обслуживанию

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268XA
Service Manual
2-42
Z = zero offset.
Zero offsets are also covered in “Zero Offset Readings” later in this chapter.
DISCHARGE Signal
The signal DISCHARGE is driven by the A3U5 A/D microprocessor (pin 59) through
one of its parallel port pins and controls the discharge of certain filter capacitors. This
line is normally left low. It is driven high during the V ac discharge mode. See “V ac
Discharge Mode” later in this chapter for more information.
Open-Thermocouple Detector
To check for an open thermocouple input, the appropriate channel is selected with the
function relays also set to the appropriate position, and the OTC circuitry is enabled. This
is done by setting the OTC_EN bit high and turning on the OTC_CLK signal, with a
frequency of 19.2 kHz. OTC_CLK is supplied by the A3U5 A/D microprocessor in the
form of the SCC3 baud rate generator (BRG3 pin). After 1.7 ms, the OTC bit is read to
determine the status of the channel. A 1 represents an open thermocouple.
After the reading, the OTC_CLK signal is turned off by setting it high. Then the
OTC_EN bit is set low.
After deselecting the channel, the measurement circuitry that has been charged by the
OTC test must be discharged. This is done by programming Stallion to apply a short
between its HI1 pin and ground, setting OTC_CLK low, and setting OTC_EN high. This
short is maintained for 500 us. After this, OTC_CLK is set high again, and OTC_EN is
set low.
Channel Measurements
The following paragraphs describe the Channel Measurement characteristics.
Reading Rates
The instrument has three reading rates: fast, medium, and slow. These measurement rates
allow you to obtain higher resolution and accuracy at the expense of slower
measurements. The instrument obtains higher resolution measurements by averaging
multiple A/D readings and/or waiting longer for signal conditioning to settle. The number
of A/D readings averaged together to obtain a single measurement is given below in
Table 2-16. Multiple A/D readings taken to average to obtain a measurement must be
taken back-to-back, without interruption, in order to obtain AC line-frequency rejection.
Note that these numbers do not apply to measurement types that do not use the A/D
converter. They also do not apply to reference balance readings (see Reference Balance
Readings).
Table 2-16. A/D Readings to Average to Obtain a Measurement
Instrument Reading Rate Fast Reading Rate Medium Reading Rate Slow
PAI 1 5 (60 Hz)
6 (50 Hz) 45 (60 Hz)
48 (50 Hz)
FAI 1 4 20 (60 Hz)
24 (50 Hz)