ANALOG MODULATION MODE
3-92
Error handling
Errors may be divided into three groups - foreground errors generally caused by a user,
background errors which represent a condition of the instrument and GPIB errors which occur
only when the unit is being controlled by a GPIB controller.
Background errors
An incorrect operating condition within the instrument automatically generates an error message to
warn the operator. For example, if the internal frequency standard should fail the message
Int.
Standard Failure
will be displayed at the top of the screen. Background errors are listed in
Table 3-2.
Foreground errors
Attempts to set the instrument to a parameter value outside its known range result in the generation
of an error message. For example, trying to select a carrier frequency above or below the specified
range results in the message
Carrier Outside Limits
being displayed at the top of the screen.
Foreground errors are cleared automatically when a correct entry is made by the user. Foreground
errors are listed in Table 3-3.
GPIB errors
Errors caused by incorrect programming are displayed at the top of the screen and may also
generate a Service Request if the relevant status registers are set. GPIB errors are listed in
Table 3-4.
Error display
Front panel
Errors are displayed as a single line of text at the top of the screen. If more than one error is
present an internal priority ordering algorithm determines which error is displayed.
GPIB
When an error occurs, its number is entered into the Error Queue. Errors are not removed from
the queue when they are cleared, but only by the ERROR? query, which returns the error at the
head of the queue, or by the *CLS command which clears the whole queue. When the queue
contains an error entry, a bit (<erb>) on the status byte is set.
The error queue has a capacity of 100 error numbers. If an error occurs while the queue is full the
last error number is replaced with 255 so that the ERROR? query returns a value of 255 to indicate
a full queue. An empty queue returns a value of 0 following an ERROR? query.
In addition to the error queue entry, the appropriate bit in the Standard Event Register will also be
set (one of <cmd>, <exe>, <dde> or <qye>). Many background errors are also reported in the
Hardware and Coupling Status Registers. For the above registers see Chap. 3-2.