Configuration and Installation
Chapter 2
SCXI-1162 User Manual
2-12
© National Instruments Corporation
In 1 is shown driven by a logic gate that is powered by the 5 to 10 V power supply. This gate
may be a TTL, CMOS, open-collector, or other logic gate capable of sinking at least 7 mA when
it outputs a logic low. When the gate outputs a logic low, it sinks current from In 1, and
channel 1 returns a logic low. When the gate outputs a logic high, no current flows through In 1
and channel 1 returns a logic high.
In 2 as shown is connected to the negative voltage of the 5 to 10 V power supply when switch S2
is closed and is disconnected when S2 is open. Thus, closing switch S2 will drive In 2 to 5 to
10 V below Vcc, causing channel 2 to return a logic low. Opening switch S2 will prevent
current from flowing through In 2, causing channel 2 to return a logic high.
In 3 is shown as unconnected; thus, no current is available to drive it below Vcc, and channel 3
will return a logic high.
The Vcc and inputs of each bank are isolated from the Vcc and inputs of any other bank. In
addition, all signals are isolated from the chassis earth ground and the internal circuitry of the
module. These isolation barriers provide isolation for voltages up to 450 VAC or VDC.
There is no isolation between signals of the same bank! The voltage at any input must never be
driven more than 10 V below its Vcc, nor may it ever be allowed to go more than 5 V above its
Vcc.
If a Vcc must be connected to a power supply of more than 10 V, you can add an additional
series resistor to an input to limit the current through that input to below 25 mA. This additional
resistor prevents the voltage at the input from being driven more than 10 V below Vcc. To select
the proper amount of series resistance and the required power rating of the series resistor, use the
following equations:
(Vcc - 10) / 24 < R
series
< (V
drop
-4) / 7
P
rated
> (Vcc)
2
/ (1000 x R
series
)
where Vcc = the maximum voltage of the power supply connected to the Vcc terminal
V
drop
= the voltage drop below Vcc required to guarantee a logic low
R
series
= the value of the series resistor in k
Ω
P
rated
= the power rating of the series resistor (in watts).
For example, if you are using a 24 V power supply, and you want the channel to return a logic
low whenever the input is driven at least 20 V below Vcc, the first equation becomes:
(24 - 10) / 24 < R
series
< (20 - 4) / 7
1.4 < R
series
< 2.28
If a resistance of 1.5 k
Ω
is used, the power rating of the resistor must be at least
(24)
2
/ (1500) = 0.38 watts. Figure 2-4 shows one way to implement this example.
