Chapter 3 Signal Connections
© National Instruments Corporation 3-9 6527 User Manual

Reducing the Forward Current for High Voltages

As input voltage increases above 5 V, the input current drawn by the 6527
(forward current IF) also rises. At 24 V, for example, current is
approximately (24 V – 1.5 V)/3 kΩ = 7.5 mA per line.
If you wish to reduce the current and power the 6527 draws—to reduce the
impact on a circuit you are monitoring, for example—you can add another
resistor in series with the 3 kΩ current-limiting resistor on the 6527. It is
recommended you choose a resistance value allowing at least 1mA to flow
through the LED. Assume a maximum drop across the LED of 1.5 V. For
example, for 24 V inputs you could use a resistance of up to
(24V – 1.5 V)/1 mA – 3 kΩ ≈ 20 kΩ for RS.
Figure 3-4. Reducing Input Current for High Voltage Signals
Solid-State Relay Outputs
On a 6527 device, I/O connector pins 51 through 98 shown in Figure 3-1
represent the terminals of the solid-state relays.

Output Channels

The output channels of a 6527 device are solid-state relays containing an
LED and two MOSFETs connected together to form a bidirectional switch.
Depending on how the load is connected to the terminals, an output can
either source or sink currents.
Figure 3 -5 shows two signal connection examples for driving a load with
these solid-state relays.
0.25 W
6527
Isolation
3 kΩ
Load
DIG+
DIG–
Supply
Isolated Ground
Rs
If
+
–