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)
Summary of Contents for 2680A
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Page 215: ...8 3 8 Schematic Diagrams Figure 8 1 Backplane PCA 2680A 4001 ...
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Page 217: ...8 5 8 Schematic Diagrams Figure 8 2 Controller System Power PCA 2680A 4004 ...
Page 223: ...8 11 8 Schematic Diagrams Figure 8 3 Digital I O PCA 2680A 4006 ...
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Page 227: ...8 15 8 Schematic Diagrams Figure 8 4 Backplane Extender PCA 2680A 4009 ...
Page 228: ...268XA Service Manual 8 16 Figure 8 4 Backplane Extender PCA cont 2680A 1009 ...
Page 229: ...8 17 8 Schematic Diagrams Figure 8 5 Extender PCA 2680A 4010 ...
Page 230: ...268XA Service Manual 8 18 Figure 8 5 Extender PCA cont 2680A 3010 ...
Page 231: ...8 19 8 Schematic Diagrams Figure 8 6 A D Supply Assembly 2680A 4031 ...
Page 232: ...268XA Service Manual 8 20 Figure 8 6 A D Supply Assembly cont 2680 1031 ...
Page 233: ...8 21 8 Schematic Diagrams Figure 8 7 PCMCIA PCA 2680A 4041 ...
Page 234: ...268XA Service Manual 8 22 Figure 8 7 PCMCIA PCA cont 2686 1041 ...
Page 235: ...8 23 8 Schematic Diagrams Figure 8 8 Output PCA 2680A 4062 ...
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Page 237: ...8 25 8 Schematic Diagrams Figure 8 9 A D PFE PCA 2640A 4003 ...
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Page 250: ...268XA Service Manual 8 38 Figure 8 10 A D FFE PCA cont 700p29_topress zip 2645A 1003 6 of 6 ...
Page 251: ...8 39 8 Schematic Diagrams Figure 8 11 Analog Input Connector PCA 2620A 4004A ...
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