Measurement Computing USB-QUAD08 - Manual (Page 26)

USB-QUAD08 User's Guide Functional Details
26
Driving digital outputs
The outputs are open-collector, effectively sinking current. The USB-QUAD08 has weak 10 kΩ resistors pulled
up to +5V with over-voltage protection. Using this default configuration may not provide adequate drive for
your application.
If minimum current is required, install a 250 Ω resistor from a digital output bit sinking from a +5V terminal.
Do not exceed 20 mA.
If a stronger drive strength is required, use an external supply with a series resistor up to 500 mA load per
digital output pin. Do not exceed 2.5 A for the device.
Counter-electro-motive force (CEMF) suppression
Counter-electromotive force is the voltage, or electromotive force, that is induced into an inductor due to an
alternating or pulsating current. CEMF is caused by a changing electromagnetic field, and is always in polarity
opposite to that of the applied voltage. The USB-QUAD08 provides a
CLMP+ screw terminal to suppress
CEMF. For CEMF protection, attach an external supply to the CLMP+ terminal directly — do not install the
supply after the series resistor.
Caution! Each output can sink up to 500 mA. Ensure that the entire device (up to 5 outputs) sinks less than
2.5 A. Alternately, all outputs can sink 300 mA.
Figure 23 below shows the digital output/timer output circuit.
Figure 23. Digital/timer output channel circuit
Triggering
You can trigger a synchronous acquisition of counter data internally with software or externally using the
XTRIG digital trigger input screw terminal.
The XTRIG input allows TTL-level triggering with latencies guaranteed to be less than 1 µs. The acquisition can
be triggered on a rising or falling edge, or on a high or low level. The trigger input is TTL logic . Latency is one
sample period, maximum. The input signal range is –0.5 V to 7 V maximum. The logic level (1 or 0) and the
rising or falling edge for the discrete trigger input are software selectable.