Measurement Computing USB-7202 - Manual (Page 14)

USB-7202 User's Guide Functional Details
14
For more information on signal connections
For general information regarding digital signal connections and digital I/O techniques, refer to the Guide to
DAQ Signal Connections (available on our website at
www.mccdaq.com/signals/signals.pdf ).
SYNC I/O
The SYNC connection is a bidirectional I/O signal that can be configured as an input (default) or an output.
 Configure as an external clock input to pace the A/D conversions from an external source. The SYNC
terminal supports TTL-level input signals of up to 50 kHz.
 Configure as an output to pace conversions on a second USB-7202.
The SYNC pin on the screw terminal is internally connected to the SYNC pin on the Trigger/Sync connector
(see page 12).
Trigger input
Use the digital trigger input ( TRIG_IN ) to start an acquisition when a rising or falling edge is detected on the
signal. This pin is internally connected to the TRIG_IN pin on the Trigger/Sync connector (see page 12).
Counter input
The CTR connection is a TTL-level input to a 32-bit event counter. The internal counter increments when the
TTL level transitions from low to high. The counter can count frequencies of up to 1 MHz.
Power output
The +5VUSER connection is a 5 volt output that is supplied by the host computer.
Caution! Do not connect to an external power supply to this terminal or you may damage the USB-7202
and possibly the computer.
The maximum total output current that can be drawn from all USB-7202 connections (power, analog, and
digital outputs) is 500 mA. This maximum applies to most personal computers and self-powered USB hubs.
Bus-powered hubs and notebook computers may limit the maximum available output current to 100 mA.
The USB-7202 alone draws 150 mA of current from the USB +5 V supply. Once you start running
applications with the USB-7202, each DIO bit can draw up to 2.5 mA. The maximum amount of +5 V current
available for experimental use, over and above that required by the USB-7202, is the difference between the
total current requirement of the USB-7202 (based on the application) and the allowed current draw of the
computer platform (500 mA for desktop computers and self-powered hubs). It shall not exceed 375 mA,
based on the Slo-Blo Fuse current rating connected to the +5V terminal.
With all outputs at their maximum output current, you can calculate the total current requirement of the USB-
7202 USB +5 V as follows:
(USB-7202 @ 150 mA) + (8 DIO @ 2.5 mA ea) = 170 mA
For an application running on a computer or powered hub, the maximum available excess current is
500 mA
− 170 mA = 330 mA. This number is the total maximum available current at the
+5VUSER screw
terminal. Measurement Computing highly recommends that you figure in a safety factor of 20% below this
maximum current loading for your applications. A conservative, safe user maximum in this case would be in
the 250 mA to 300 mA range.
Since some laptop computers running on battery power only allow up to 100 mA, the USB-7202 may be
above that allowed by the computer. In this case, you must either purchase a self-powered hub or operate the
laptop computer from an external power adapter.
Ground
The analog ground ( AGND ) terminals provide a common ground for all analog channels. The digital ground
( GND ) terminals provide a common ground for the digital, trigger, counter, and sync channels and the power
terminal.