Measurement Computing DaqBoard 3000USB Series - Manual (Page 37)

USB2.0 versus USB1.1
Connecting a DaqBoard/3000USB Series board to a USB1.1 port or hub will result in lower transfer speed
which may not support continuous data collection at high scan rates. Note that Hi-Speed (USB2.0) ports
are at least forty times faster than the earlier Full-Speed (USB1.1) versions.
When the host computer has a board with USB 2.0 ports, an “Enhanced” USB controller can be found in
the Device Manager. The Device Manager will also show two other USB controllers. This is due to the
fact that USB2.0 circuitry includes 3 chips [one for the actual USB2.0 capable devices and two for
backward USB1.1 compatibility]. Thus a USB 2.0 motherboard can host any USB device (version 2.0 or
lower), assuming there are no defects with the board, system, and/or device.
Notes on USB Hubs:
•
USB 1.1 (obsolete) hubs will work on USB 2.0 ports, but cannot utilize USB 2.0 capabilities.
•
Hi-Speed and Full/Low-Speed USB devices can coexist on USB 2.0 hubs.
•
USB 2.0 hubs can be used on computers with USB 1.1 ports, but will not exhibit USB 2.0
capabilities.
•
Minimize hub use and keep USB cables as short as possible.
•
Regardless of the USB hub or port used, if power to the DaqBoard/3000USB Series board is
insufficient, connect a TR-2 power adapter to the unit’s External Power jack.
•
Only self-powered hubs can supply sufficient power (500 mA at 5V nominal). Verify that the
AC-to-DC power supply for the self-powered hub can supply at least 2.1 amps at 5 volts.
•
In general, do not use more than three DaqBoard/3000USB systems per one self-powered hub.
Triggering
Triggering can be the most critical aspect of a data acquisition application. The DaqBoard/3000USB Series
supports a full complement of trigger modes to accommodate any measurement situation.
Hardware Analog Triggering. TheDaqBoard/3000USB Series uses true analog triggering, whereby the
trigger level programmed by the user sets an analog DAC, which is then compared in hardware to the
analog input level on the selected channel. The result is analog trigger latency which is guaranteed to be
less than1.3 µs. Any analog channel can be selected as the trigger channel. The user can program the
trigger level, as well as the rising or falling edge and hysteresis.
When the starting out analog input voltage is near the trigger level, and you are
performing a rising [or falling] hardware analog level trigger, it is possible that the
analog level comparator will have already tripped, i.e., to have tripped before the sweep
was enabled.
If this is the case, the circuit will wait for the comparator to change state. However, since the
comparator has already changed state, the circuit will not see the transition.
Solution: (1) Set the analog level trigger to the desired threshold.
(2) Apply an analog input signal that is more than 2.5% of the full-scale range away from
the desired threshold. This ensures that the comparator is in the proper state at the
beginning of the acquisition.
(3) Bring the analog input signal toward the desired threshold. When the input signal is at
the threshold (± some tolerance) the sweep will be triggered.
(4) Before re-arming the trigger, again move the analog input signal to a level that is more
than 2.5% of the full-scale range away from the desired threshold.
Example: o an engineer is using the ±2V full-scale range (gain = 5)
o he desires to trigger at +1V on the rising edge
o he sets the analog input voltage to an initial start-value which is
less than +0.9V (1V – (2V * 2 * 2.5%)).
DaqBoard/3000USB Series User’s Manual 988093 Device Overviews 1-11