Keithley 6512, Инструкция по эксплуатации, Страница: 48 / 156

Operation
2-28
The instrument will then output readings over the IEEE-
488 bus at the selected rate.
5. The storage rate in r=0 and r=1 may be affected if the in-
strument is in auto-range and a range change occurs.
Typically, it takes about 350msec per range change.
Minimum/maximum operation
Minimum/Maximum operation is essentially separate from
data store except for the fact that both are enabled or disabled
by the ON/OFF button. Thus, the minimum and maximum
data points are continuously updated with each triggered
conversion as long as the ON/OFF LED is on. Note that only
range, not function, is indicated when reading maximum and
minimum values. Maximum and minimum values can be ob-
tained during the recall process, as indicated in step 4 above.
2.12 External triggering
The Model 6512 has two BNC connectors on the rear panel
associated with instrument triggering. The EXTERNAL
TRIGGER INPUT allows the instrument to be triggered by
other devices, while the METER COMPLETE OUTPUT al-
lows the instrument to trigger other devices.
2.12.1 External trigger input
The Model 6512 may be triggered on a continuous or one-
shot basis. For each of these modes, the trigger stimulus will
depend on the selected trigger mode, which is further de-
scribed in Section 3. In a continuous trigger mode, the instru-
ment takes a continuous series of readings. A trigger
stimulus in continuous aborts a reading in progress and trig-
gers a new series of readings. In a one-shot mode, only a sin-
gle reading is taken each time the instrument is triggered.
The EXTERNAL TRIGGER INPUT requires a falling-edge
pulse at TTL logic levels, as shown in Figure 2-20. The low
logic level should be between 0-0.8V, and the high level
should be 2-5V. The minimum pulse width for reliable trig-
gering is approximately 10µsec. Connections to the rear pan-
el EXTERNAL TRIGGER INPUT jack should be made with
a standard BNC coaxial cable. If the instrument is in the ex-
ternal trigger mode, it will be triggered to take readings while
in either a continuous or one-shot mode when the negative-
going edge of the external trigger pulse occurs.
Figure 2-20
External trigger pulse specifications
To use the external triggering, proceed as follows:
1. Connect the external trigger source to the rear panel
BNC EXTERNAL TRIGGER INPUT connector. The
shield (outer) part of the connector is connected to digi-
tal common. Since an internal pull-up resistor is includ-
ed, a mechanical switch may be used. Note, however,
that de-bouncing circuitry will probably be required to
avoid improper triggering.
2. Place the instrument in the one-shot trigger mode by
pressing SHIFT and then TRlG in that order. The instru-
ment will indicate that it is in the one-shot mode by illu-
minating the SGL indicator.
3. To trigger the instrument, apply a pulse to the EXTER-
NAL TRIGGER INPUT. The instrument will process a
single reading each time the pulse is applied. Note that
the instrument may also be triggered by pressing TRIG.
4. To return the instrument to the continuous mode, press
the SHIFT and TRIG buttons in sequence.
NOTES:
1. External triggering can be used to control the fill rate in
the data store mode. See paragraph 2.11 for details.
2. The Model 6512 must be in the appropriate trigger mode
to respond to external triggering (the unit will be in this
mode upon power-up). See Section 3 for details on pro-
gramming trigger modes.
TRIGGERS ON
LEADING EDGE
TTL HIGH
(2V-5V)
TTL LOW
(
≤
0.8V)
10
µ
s
MINIMUM