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Isolators
Data is transferred serially between the primary interface and the secondary interface via optical isolators. As described
above, the primary interface circuits are referenced to chassis ground while the secondary interface and input power circuits
are reference to load common. Neither of the load's input terminals ( + or - ) can exceed
± 240Vdc from ground.
The trigger signal is also transferred from the primary interface via an optical isolator. The trigger signal can be used to
control data transfers into the DAC circuits and can also be used in transient operation.
Secondary Interface
The secondary interface circuit translates the serial data received from the primary interface into a parallel data bus and
other control signals. The data bus and control signals are sent to the power control circuits to control the input power stages
in accordance with the programmed parameters. Status and measurement information is also read back to the GP-IB
controller and/or the front panel display via the secondary and primary interfaces.
DACs and Slew Rate Control
Programmable main and transient DAC circuits convert the programmed data into an analog signal (PROG) that controls the
conduction of the input power stages. Depending upon the mode of operation, the main DAC circuit converts the
programmed value of current, resistance, or voltage into an analog signal to control the input power stages. The conversion
can be initiated by a GP-IB command or by a trigger (GP-IB or external).
The transient DAC circuit and a programmable generator allow transient operation in the selected mode. Transient
operation causes the input power stages to switch between two load levels. Transient operation can be programmed at a
continuous rate or can be triggered (programmed trigger or an external trigger signal) to produce a transient pulse or to
switch between two load levels. Programmable slew rate control circuits allow a controlled transition from one load setting
to another.
An external programming signal can also be used to control conduction of the input power stages in the CC or CV mode. A
0 to 10V external programming signal corresponds to the 0 to full scale input rang in the CC or CV mode. The external
signal is combined with the programmed values from the main and transient DAC circuits.
A readback DAC circuit returns the input current, input voltage, and heatsink temperature values to the secondary
microprocessor. The DAC circuit along with a comparator circuit are controlled by the secondary microprocessor to
successively approximate the value of the monitored signal to 12-bit resolution. The readback DAC and comparator circuit
also return a test signal to the microprocessor during self test to determine if the DAC circuits are operating properly.
CC/CV Control
Depending upon which operating mode (and range in the CR mode) is selected, either the CC or the CV loop controls the
conduction of the input power stages. If the CC or CR (middle or high resistance ranges only) mode is selected, the CC loop
controls the conduction of the input power stages according to the selected mode and the programmed value of current or
resistance. A range control signal is sent to the CC control circuit to provide the proper scaling for the low and high current
ranges or the middle and high resistance ranges.
If the CV or CR (low resistance range) mode is selected, the CV loop controls the conduction of the input power stages
according to the selected mode and the programmed value of voltage or resistance.
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