Counter Input; Power Output; Ground - Measurement Computing USB-7204 User Manual

USB-7204 User's Guide
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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 pin is internally connected to the SYNC pin on the Trigger/Sync connector (see page 11).
SYNC

Counter input

The CTR connection is a TTL-level input to a 32-bit event counter. Refer to Figure 4 on page 12 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
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
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
350 mA to 375 mA range (fuse-rated).
Since laptop computers typically allow up to 100 mA, the USB-7204 in a fully-loaded configuration may be
above that allowed by the computer. In this case, you must determine the per-pin loading in the application to
ensure that the maximum loading criteria is met. The per-pin loading is calculated by simply dividing the +5 V
by the load impedance of the pin in question.

Ground

The analog ground ( AGND
( ) terminals provide a common ground for the digital, trigger, counter, and sync channels and the power
GND
terminal.
connection is a 5 volt output that is supplied by the host computer.
) terminals provide a common ground for all analog channels. The digital ground
16
Functional Details
screw
+5V output
user