Measurement Computing USB-1024LS - Manual (Page 14)

USB-1024LS User's Guide Functional Details
Power terminals
The PC +5 V connection (pin 30) is on the bottom screw terminal of the USB-1024LS. Refer to the pinout
diagram on page 3-2 for the location of this pin. This terminal draws power from the USB connector. The +5 V
screw terminal is a +5 volt output that is supplied by the computer.
Caution! The +5 V terminal is an output. Do not connect to an external power supply or you may damage
the USB-1024LS and possibly the computer.
The maximum total output current that can be drawn from all USB-1024LS connections (power 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.
Just connecting the USB-1024LS to your computer draws 18 mA of current from the USB +5 V supply. Once
you start running applications with the USB-1024LS, each DIO bit can draw up to 2.5 mA. The maximum
amount of +5V current available for external use, over and above that required by the USB-1024LS, is the
difference between the total current requirement of the USB-1024LS (based on the application), and the
allowed current draw of the PC platform (500 mA for desktop PCs and self-powered hubs, or 100 mA for
bus-powered hubs and notebook computers).
With all outputs at their maximum output current, you can calculate the total current requirement of the USB-
1024LS USB +5 V as follows:
(USB-1024LS @ 18 mA) + (24 DIO @ 2.5 mA ea) = 78 mA
For an application running on a PC or powered hub, this value yields a maximum user current of 500 mA−78
mA = 422 mA. This number is the total maximum available current at the PC+5V screw terminals.
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 330 mA.
Since laptop computers typically allow up to 100 mA, the USB-1024LS 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 terminals
The 8 ground ( GND ) connections are identical, and provide a common ground for all USB-1024LS functions.
Refer to the pinout diagram on page 3-2 for the location of the GND terminal pins.
Counter terminal
Pin 20 ( CTR ) is the input to the 32-bit external event counter. Refer to the pinout diagram on page 3-2 for the
location of this pin. The internal counter increments when the TTL levels transition from low to high. The
counter can count frequencies of up to 1 MHz.
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