Astronics OMNIBUS II NI PXIe, User Manual

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OMNIBUS II FEATURES
OmniBus II NI PXIe User Manual 4-9
The processes that are configured to be triggered by an external trigger can be
triggered through software using the
BTICard_CardTriggerEx
function.
This is useful for software testing and does not require external trigger equipment.
4.5 1553 Avionics Discretes
OB2 modules that have MIL-STD-1553 channels (see Sections 6.2) also have 16
avionics shunt discretes (hereafter referred to as 1553 discretes). While these
discrete I/O are simpler than those on the discrete module, they can still signal and
detect events, determine status, and drive loads. The 1553 discretes may also be
used as general purpose digital I/O and have a wide range of avionics and general-
purpose applications. You can configure each 1553 discrete I/O pin as both a shunt
input and a shunt output. When used as an output, you can verify the status of a
pin by reading the input for the pin.
4.5.1 Shunt Inputs
A shunt input circuit pin is pulled up to a voltage source through a resistor. A load
resistance applied between the pin and ground will shunt current from the source
and generate a voltage at the pin. The pin voltage is compared with a reference
voltage for input state detection. There are two defined states: the open state in
which a high impedance is applied to the pin, and the ground state in which a low
impedance is applied to the pin.
The OB2 1553 discrete shunt input circuit, illustrated in Figure 4.2, has a 9-
kΩ
pull-up resistor to a 5-V source. An isolation diode provides protection against
over voltage at the pin. A load resistance connected between the input pin and
ground will shunt current from the 5-V source, through the forward-biased diode
and the 9-
kΩ resistor. A series resistor limits current as a voltage is generated
across the load that is compared to a reference voltage produced by an identical
configuration. This results in a 3.25-V switching voltage.
Figure 4.2—OmniBus II Discrete Shunt Input Circuit
5 k
Ω
5 V
9 k
Ω
R
app
5 k
Ω
3.25V
5 V
9 k
Ω