Measurement Computing USB-7204 - Manual (Page 16)

USB-7204 User's Guide Functional Details
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When configured for input, the digital I/O terminals can be used to detect the state of any TTL level input.
For more information on digital signal connections
For more information on digital signal connections and digital I/O techniques, refer to the Guide to DAQ Signal
Connections (available on our web site at
www.mccdaq.com/signals/signals.pdf ).
Trigger input
The digital trigger input ( TRIG_IN ) lets you start an acquisition when a signal's rising or falling edge is detected.
This pin is internally connected to the TRIG_IN pin on the Trigger/Sync connector (see page 11).
SYNC I/O
The SYNC terminal is a bidirectional I/O signal that can be configured as an input or an output (default):
 Configure as an external clock input to externally pace the A/D conversions from an external source or
another USB-7204. The SYNC terminal supports TTL-level input signals of up to 50 kHz.
When used as a clock input, the SYNC pin operates in one of two modes – slave or gated slave.
o In slave mode, the USB-7204 ignores the first clock pulse in order to ensure adequate setup time. Use
this mode when the USB-7204 is being paced from a continuous clock source, such as a generator.
o In gated slave mode, it is assumed that the clock signal is held off for an adequate amount of time for
setup to occur. No clock pulses are ignored. Use this mode when the source of the external clock is
another USB-7204.
 Configure as an output to pace the conversions on another USB-7204.
When configured as an output, the internal A/D pacer clock is sent to the screw terminal. You can use this
signal as a clock input to a second USB-7204 by connecting it to the SYNC pin and Trig/SYNC connector
of the second device.
The SYNC pin is internally connected to the SYNC pin on the Trigger/Sync connector (see page 11).
Counter input
The CTR connection is a TTL-level input to a 32-bit event counter. Refer to Figure 4 on page 13 for the location
of this pin. 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 output 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-7204
and possibly the computer.
The maximum total output current that can be drawn from all USB-7204 connections (power, analog and digital
outputs) is 420 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.
Just connecting the USB-7204 to your computer draws 80 mA of current from the USB +5 V supply. Once you
start running applications with the USB-7204, each DIO bit can draw up to 2.5 mA, and each analog output can
draw 15 mA. The maximum amount of +5 V current available for experimental use, over and above that
required by the USB-7204, is the difference between the total current requirement of the USB (based on the
application) and the allowed current draw of the computer platform (500 mA for desktop computers and self-
powered hubs, or 100 mA for bus-powered hubs and notebook computers). 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-
7204 USB +5 V as follows:
(USB-7204 @ 80 mA) + (16 DIO @ 2.5 mA ea) + (2 AO @ 15 mA ea ) = 150 mA
For an application running on a computer or powered hub, the maximum available excess current is
500 mA
− 150 mA = 350 mA. This number is the total maximum available current at the
+5V
user
output screw