Measurement Computing USB-1208LS - Manual (Page 14)

USB-1208LS User's Guide Functional Details
14
Refer to the schematic shown in Figure 8. If the switch is set to the +5 V input, Port A0 reads TRUE (1). If you
move the switch to GND, Port A0 reads FALSE.
Figure 8. Schematic showing switch detection by digital channel Port A0
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 TRIG_IN terminal is an external digital input that you can configure for either TTL level high or low.
Counter input
The CTR terminal is a 32-bit event counter that can accept frequency inputs up to 1 MHz. The internal counter
increments when the TTL levels transition from low to high.
Calibration output
The CAL terminal is an output used only to calibrate the device. Calibration is software-controlled via InstaCal.
Power output
The PC +5V terminal is a 5 volt output that is supplied by the computer. You can use this terminal to supply
power to external devices or circuitry.
Caution! The
PC +5V terminal is an output. Do not connect to an external power supply or you may damage
the USB-1208LS and possibly the computer.
The maximum total output current that can be drawn from all USB-1208LS 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.
Just connecting the USB-1208LS to your computer draws 20 mA of current from the USB +5 V supply. Once
you start running applications, each DIO bit can draw up to 2.5 mA, and each analog output can draw 30 mA.
The maximum amount of +5 V current available for experimental use, over and above that required by the
device, is the difference between the total current requirement of the USB device (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, the total current requirement of the USB +5 V is:
(USB-1208LS @ 20 mA) + (16 DIO @ 2.5 mA ea) + (2 AO @ 30 mA ea ) = 120 mA
For an application running on a PC or powered hub, the maximum available excess current is 500 mA−120 mA
= 380 mA. This number is the total maximum available current at the PC+5V screw terminals. Cole-Parmer
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 300-320 mA range.
Since laptop computers typically allow up to 100 mA, the USB-1208LS 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 GND connections are identical and provide a common ground for all device functions.