RedLab 1008 User's Guide
Functional Details
22
Power terminals
The
PC +5V
connections on the screw terminal draw power from the USB connector. The +5 V screw terminal
is a 5 volt output that is supplied by the computer.
Caution!
The PC +5V terminals are outputs. Do not connect an external power supply to a +5 V screw
terminal, or you may damage the device and possibly the computer.
The maximum total output current that can be drawn from all RedLab 1008 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 available output current to 100 mA.
Just connecting the RedLab 1008 to your computer draws 20 mA of current from the USB +5V supply. Once
you start running applications with the device, 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 to the user is the difference between the
total
current requirement
of the PMD (based on the application), and the
allowed current draw
of the PC platform
(again, 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
1008 device's USB +5 V as follows:
(RedLab 1008 @ 20 mA) + (28 DIO @ 2.5 mA) + (2 AO @ 30 mA) =150 mA
For an application running on a PC or powered hub, this value yields a maximum user current of
500 mA
−
150 mA = 350 mA. This number is the total maximum available current at the PC +5 V 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 in the
280 mA range.
Since laptop computers typically allow up to 100 mA, the RedLab 1008 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 +5V
by the load impedance of the pin in question.
Ground terminals
There are nine identical ground connections that provide a common ground for all RedLab 1008 functions.
Refer to the
pinout diagrams
on page 18 for the location of the
GND
terminal pins.
Calibration terminal
The
CAL
connection on the output terminal provides a calibration reference voltage. This terminal should only
be used during calibration of the RedLab 1008 device.
Calibration of the RedLab 1008 is software-controlled via
InstaCal
.
Testing terminal
The
TST
terminal is reserved for factory testing only.
Summary of Contents for RedLab 1008
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