Measurement Specialties PMD-1024HLS - Manual (Page 16)

PMD-1024HLS User's Guide Functional Details
Port A0
+GND +5 V
Figure 3-4. 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 web site at
www.mccdaq.com/signals/signals.pdf ).
Power terminals
The USB +5 V connection (pin 30) is on the bottom screw terminal of the PMD-1024HLS. Refer to the
pinout diagram on page 3-2 for the location of this terminal. This terminal draws power from the USB
connector. The +5 volt screw terminal is a +5 volt output that is supplied by the connected computer.
Caution! The USB +5 V terminal is an output. Do not connect to an external power supply or you
may damage the PMD-1024HLS and possibly the computer.
Just connecting the PMD-1024HLS to your computer draws 60 mA of current from the USB +5 V
supply. Once you start running applications with the PMD-1024HLS, each DIO bit can draw up to
15 mA. The maximum amount of +5 V current available for external use, over and above that required by
the PMD-1024HLS, is the difference between the total current requirement of the PMD-1024HLS (based
on the application), and the allowed current draw of the computer platform.
The following power limits depend on whether you are sourcing current out of the PMD-1024HLS, or
you are sinking current into the PMD-1024HLS.
Output (source) current limits: desktop computers and self-powered hubs
The maximum allowed current draw for a typical desktop computer and self-powered hub is 500 mA. For
an application running on a PC or self-powered hub, this value yields a maximum user current of 500 mA
− 60 mA = 440 mA. Since each DIO line can output 15 mA maximum, 24 DIO × 15 mA = 360 mA
(maximum rating for the chips).
Output (source) current limits: notebook computers and bus-powered hubs
The maximum allowed current draw for a typical notebook computer and bus-powered hub is 100 mA.
Because the PMD-1024HLS needs 60 mA from the USB +5 V supply, this leaves you with only 40 mA.
If you want to use 15 mA per DIO line, you can only do so with two DIO lines. To overcome this
limitation, an external self-powered hub can be used to supply the proper current level required for full
output capability as noted above.
Output (sink) current limits
Each PMD-1024HLS DIO output can sink up to 64 mA. For an individual line, this is not a problem. But
if you were to apply this maximum load to all 24 DIO lines, you would damage the PMD-1024HLS
because the total amount of current is well over the 440 mA maximum (24 DIO × 64 mA = 1536 mA ≈
1.54 A).
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