National Instruments DAQ 6527

Chapter 3 Signal Connections

Figure3-6 shows a signal connection example for both sinking and

sourcing current. The example shows a TTL-level application with a supply

voltage of +5 V. The 6527 provides sink current when the relay is closed.

Resistor RL provides source current when the relay is open.

When the relay is open, little current flows through the resistor and the

output voltage is close to 5 V, a logic high. When the relay is closed, current

flows through the load and the output voltage is close to 0 V, a logic low. If

isolation is not a concern, you can use the +5 V line from the 6527 device

in place of the external +5 V supply.

Choose a value of RL small enough to provide the source current you need

but large enough to avoid reducing sink current or consuming unnecessary

power. For many TTL-level applications, a value of approximately

RL=5kΩ works well. This gives a source current at 2.8 V of

(5V–2.8V)/5kΩ=440 µA. The sink current at 0.5 V is then at least

(0.5 V / 35 Ω) –(5V–0.5V)/5kΩ=13.4 mA.

Figure 3-6. Signal Connections for Driving TTL Voltages

The maximum power ratings for the output channels on a 6527 device are

as follows:

• Maximum DC voltage across the terminals (VOUT) = 60 VDC

• Maximum AC voltage across the terminals (VOUT) = 30 VRMS

(42 VPeak)

• Maximum current (If) = 120 mA1

1 With all relays carrying 120 mA and all inputs driven to 28 V, the total power dissipation can approach 20 W. The maximum

switching capacity in PCI and CompactPCI systems must be derated according to the ambient temperature. (The PXI chassis

has built-in fans to handle 25 W per slot.)

+5 V

RL= 5 kΩ

VOUT

Isolation

Isolated

Ground

Digital

Logic

390 Ω

To External +5 V Supply

35 Ω

6527