Monitor Scan - Keithley 2701 User Manual

Model 2701 User's Manual Scanning 7-35

Monitor scan

For this example, channel 101 of the Model 7700 is used to monitor temperature. When
the temperature reading reaches 30°C, it will start the scan. For this 4-channel scan,
channel 101 measures temperature, while channels 102, 103, and 104 measure DCV.
This example uses the channel average feature to measure temperature. With channel
average enabled, two temperature measurements will be taken; one at channel 101 and
another at its paired channel (111). The two measured readings are then averaged to yield
a single reading. It is this averaged temperature reading that will start the scan when it
reaches 30°C. Refer to Section 5 for details on channel average.
Two type K thermocouples are used to measure temperature. Since the internal cold
(reference) junction of the Model 7700 is being used, the thermocouples can be connected
directly to the screw terminals of the switching module.
Operation
A simplified model of monitor scan operation is shown in Figure 7-5, while the procedure
steps and programming commands are listed in Table 7-3.
In Figure 7-5, notice that there are two modes of operation. While in the monitor mode,
the Model 2701 continuously performs temperature measurements. Keep in mind that
channel average is being used. Therefore each temperature reading is the average of two
temperature measurements (one on channel 101 and one on channel 111). As long as the
average temperature reading remains below 30°C, the instrument will remain in the
monitor mode.
When the temperature reading reaches 30°C, the Model 2701 switches over to the scan
mode. Figuratively speaking, it is as if a "finger" presses the SCAN key when the monitor
detects that the average temperature is at or above 30°C.
The instrument is configured to scan four channels. The monitor TEMP channel reading
and three DCV channel readings are stored in the buffer. After the fourth channel is
measured, operation returns to the monitor mode to again measure temperature. Note that
if the average temperature is still at or above 30°C, the "finger" will again press SCAN to
start the scan.