©
National Instruments Corporation
13
NI 6520/6521 User Guide
Optically Isolated Differential Inputs
The input lines on the NI 6520/6521 device provide 8 lines of optically
isolated digital input. These inputs contain an AC input photocoupler and a
resistor for current limiting. The maximum input voltage (VIN) on these
lines is 30 VDC.
(NI 6520 Only)
The input (VIN) range on the channels is –30 VDC to
+30 VDC, for signals connected between P0.
X
+ to P0.
X
–, where
X
is
line <0..7>.
Sensing DC Voltages
The NI 6520/6521 can sense DC signals from 11 V to DC power supply
levels up to 30 V.
Input voltages between –4 V to +4 V produce a logic low. Input voltages
between –30 V to –11 V or b11 V to +30 V produce a logic high.
The logic state of signals between –11 V to –4 V and +4 V to +11 V is not
defined. This corresponds to standard industrial 24 V logic thresholds.
There should never be more than a 30 V difference in P0.
X
+ and P0.
X
–
terminals. Exceeding 30 V differential voltage could cause damage to the
photocouplers.
(NI 6521 Only)
The 30 V differential voltage can have up to ±120 V
common-mode voltage compared to chassis/digital ground of the NI 6521
device.
12, 30, 14, 32, 16, 34, 18,
36
P0.<7..0>+
Isolated input ports, positive terminals
—Each of
these terminals serves as the signal input. You can
connect either end of the input signal to it, regardless
of whether the level is positive or negative.
13, 31, 15, 33, 17, 35, 19,
37
P0.<7..0>–
Isolated input ports, negative terminals
—Each of
these terminals serves as the signal input. You can
connect either end of the input signal to it, regardless
of whether the level is positive or negative.
Table 3.
NI 6520/6521 Signal Descriptions (Continued)
Pin
Signal Name
Description