Fluke 8842a instruction manual Overrange Indication, Diagnostic Self-Tests, Ranging, Autorange

8842A

Instruction Manual

2-12. Overrange Indication

An input is overrange if it exceeds the full scale of the selected range. In most ranges, the 8842A indicates an input is overrange by lighting the OVER annunciator and showing a "1" on the display. (See Figure 2-8.) The sign, the position of the decimal point, and the other annunciators are not affected.

As a safety feature, the 8842A treats the 1000V dc and 700V ac rangesdifferently. In these ranges, the 8842A indicates when the input exceeds the input overload limit of 1000V dc or 700V ac, respectively, by lighting the OVER annunciator and flashing the display. Readings are still displayed.

2-13. Diagnostic Self-Tests

The 8842A features diagnostic self-tests which check both the digital and analog circuitry in the instrument. The self-tests consist of 21 analog tests followed by the in-guard program memory, calibration memory, and display self-tests. To initiate the self-tests, press the SRQ button for 3 seconds. The instrument can be stopped in any of the test configurations by pressing the SRQ button while the test number is displayed. Press any button to continue the tests.

During the test, the TEST annunciator lights, and the numeric field displays the number of each analog test as it is performed. Then all display segments light up while the instrument performs the in-guard program memory, calibration memory, and display self- tests. The 8842A then returns to the power-up configuration. The self-tests are described in greater detail in the Maintenance section.

NOTE

The inputs must be left disconnected while the self-tests are performed or the 8842A may indicate that errors are present.

If the 8842A detects an error, it displays an error message for about 2-1/2 seconds. (Error codes 01 through 29 correspond to the self-tests.) If self-test errors are displayed even when the input terminals are disconnected, there may be a hardware problem in your 8842A. In that event, refer to the Maintenance section or contact your local Fluke representative.

2-14. Ranging

Measurement ranges can be selected using either autorange (by pressing the AUTO button) or manual range (by pressing another range button). The 8842A displays explicit units in every range, so that the display may be read directly.

2-15. AUTORANGE

In autorange, the 8842A goes to a higher range when the input exceeds full scale (199999 counts), and goes to a lower range when the input falls below 9% of full scale (18000 counts). While the instrument changes range, the numeric field on the display is blanked until a new reading is completed. However, the decimal point and units annunciators always indicate what range the instrument is in.

Pressing the AUTO button when the instrument is already in autorange toggles the 8842A from autorange to manual range. This causes the instrument to remain locked in the present range.

The 8842A autoranges up to the highest ranges in all functions, down to the 200 mV range in the VDC and VAC functions, and down to the 200Ω range in the ohms functions. To select the 20 mV dc, 20Ω, or 200 mA dc range, press the respective range button (or send the respective range command, if using the IEEE-488 option).

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