User Manual

RedLab 1024LS User's Guide Functional Details
15
+5V+GND
Port A0
Figure 6. 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 Signal
Connections (available on our RedLab CD (root directory).
Power terminals
The PC +5 V connection (pin 30) is on the bottom screw terminal of the RedLab 1024LS. Refer to the pinout
diagram on page 14 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 RedLab 1024LS and possibly the computer.
The maximum total output current that can be drawn from all RedLab 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 RedLab 1024LS to your computer draws 18 mA of current from the USB +5 V supply.
Once you start running applications with the RedLab 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 RedLab
1024LS, is the difference between the total current requirement of the RedLab 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 RedLab
1024LS USB +5 V as follows:
(RedLab 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 RedLab 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.