HP 8642A Operating Manual page 49

Model 8642A/B Operation
You can read only the first Parameters Changed message if more than one of each of these
has occurred since either the Status Byte was cleared or since the last time "OC" was
executed. Then reading a message using "OC" will enable you to read out only the first
Parameters Changed message.
Following is a programming example in BASIC for outputting an Execution Error (OE):
Output either an Execution Error code number or message string from the Signal
Generator (address 19) and reset bit 2 of the Status Byte:
To output the message code number:
10 OUTPUT 719; "OE"
20 ENTER 719; A
To output the message string:
10 OUTPUT 719; "OE"
20 ENTER 719; A ! Dummy read of the message code number
30 ENTER 719; A$
The above examples can also oe modified to apply to an Output Parameters Changed (OC)
message if you send "OC" instead of "OE". Sending either "OE" or "OC" alone (program line
10 above) will reset the appropriate status bit (unless RQS is true, in which case the Status
Byte must be read again to clear the status bit)
Note that you should not execute an instrument function between setting up the
instrument for data output and reading the data from the Signal Generator. To illustrate
this consideration, refer to the "OE" example above. If you were to insert a line 1 5 that set
an instrument function such as AM, the data received from the Signal Generator will be
the display data, not the Execution Error message code, because "AM" would have
superceded the "OE" data output preparation.
Hardware Errors
Hardware Error messages are handled differently than Execution Error or Parameters
Changed messages since more than one message can be present. These messages are stored
as a queued up list with a maximum number of 22 messages. (Read about queued up
messages on page 3-180). To properly retrieve all the queued up messages, follow the
"OH" command with a software loop as shown in the sample program in Table 3-6 (page
3-23).
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