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Feeding
an External Signal to the PulseBlaster
The voltages into the input pins of the PulseBlaster
must be positive-only and must not exceed the maximum
voltage value. Please see the manual of your board
for more information.
If the external driving source device is far from the
board and a long cable is used to connect to the input
pins, the cable must be terminated (at the end) with
resistance that equals the characteristic impedance of the
cable. If the line is fed directly to the PulseBlaster
without a terminating resistor, the voltage into the
PulseBlaster board may be greater than what is measured
with an oscilloscope.
For example, if the external signal source has a 50 ohm
output impedance, and the measured voltage is 2 V at the
end of the cable on a 50 ohm resistor, the voltage will be
4 V or so if connected to the PulseBlaster without a
terminating resistor, which will damage the board.
One method of implementation is to create a box that has
an internal CMOS inverter. The external signal connects to
the input of the box and is terminated by an appropriate
resistor. The output of the box can mount directly to the
input pin and corresponding ground of the PulseBlaster.
Using this method, the setup would look similar to the
figure below.
Figure 1: Example diagram on how to
feed an external signal to the PulseBlaster.
The drawing is created for reference and is not exact. The
external matching box does not need to be mounted directly
to the PulseBlaster connector. The box can be connected to
the PulseBlaster using a one-to-one cable that should not
be longer than 3 feet. The PulseBlaster circuitry in
Figure 1 shows the circuitry for the HW_Trigger and
HW_Reset pin but the general concept applies to the
HW_Trigger_H and HW_Reset_H pins, and also using an
external clock source.
The external signal source will need to have sufficient
current capabilities to drive the CMOS inverter over a 50
ohm load. The voltage sourcing the CMOS gate cannot be
more than the maximum input voltage of the PulseBlaster,
which is typically 3.3 V (please see the manual of your
board for more information), though 3 V will be sufficient
for logical high.
The box device has an inverter which makes it simple
to interface with input pins that require
"high-low-high" signals such as the HW_Trigger and
HW_Reset pins (see your product's manual for more
information). A simple brief pulse from the driving
source will hardware trigger or reset the PulseBlaster,
depending on which hardware pin the box is connected to.
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