HP 8340A Operating Manual page 220

2. The 8340A is in local mode but the program is
still running. Thus, if any front panel key is
pressed, an interruption occurs and the prompt
"FRONT PAN EL SRQ" is displayed and the pro­
gram execution goes back on line, waiting for
another interrupt. Note the 8340A is still in
LOCAL.
3. An "RF UNLOCKED" interrupt may be gen­
erated by disconnecting the jumper that sup­
plies the FREQUENCY STAN DARO Reference.
This jumper is connected between INT and EXT
BNC connectors of the frequency standard
input/output on the rear panel of the 8340A.
4. An SRQ ls also obtained by adjusting the power
level with the knob until the power becomes
unleveled. Then an SRQ occurs and an "OUT­
PUT IS UN LEVELED" prompt is displayed on
9826A CRT. Reduce the power level to return to
leveled power and observe another SRQ with an
"OUTPUT IS LEVELED" prompt displayed on
9826A CRT.
As bit 2 of the 8340A Status Byte shows, an SRQ
occurs when a change in the extended status
byte occurs. Thus, an SRQ is detected for a
change either from the leveled or from the
unleveled state.
Note that whatever the origin of the SRQ, the
current status of both bits 4 and 6 of the exten­
ded status byte is displayed.
NOTE
When an interruption occurs, this information
is sent to the controller. With a priority pre­
determined in the software, the controller will
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Scans by HB9HCA and HB9FSX
read the Status of the 8340A. Thus, the con­
troller does not respond instantaneously to
the Service Request. During this delay, the
cause of the SRQ may have disappeared. (See
figure 5a.) To avoid having the controller get
an SRQ with "no information", on some
functions of the extended status byte the bit
corresponding to the function requiring the
service is latched even if the cause of the SRQ
disappears before the controller has read the
extended status byte. These functions are:
overmodulation (bit 1), RF unlocked (bit 4),
power failure (bit 5) and RF unlevelled (bit
Figure 5 shows how this latching works via
two examples of output level power vari­
ations. In example (a) an SRQ is sent (t1)
when the RF output goes "unleveled". But the
RF output is back to "leveled" before the con­
troller has read the status of line 3 (bit
the extended status byte). Then the controller
reads the latched information and in so doing,
clears it (t2) so the user knows that an
unleveled output occured between t1 and t2.
To read from the status byte the current value
of bit 6, it must be read twice (t3). In this
case, the user then learns that the output is
back to being leveled. In example (b), the
controller detects an unleveled state from the
latched bit, and in the process of reading the
bit, clears it. But the latched bit returns to
"unleveled" because the current state of the
RF output is still unleveled. So, if the exten­
ded status byte is read a second time, the
current state (unleveled) is measured.
On line 190, a Clear Status bytes statement (CS) is
used to clear both status bytes. Then, they are read
again to learn the current state of the 8340A.
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