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

USB-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 +5V connection (pin 30) is on the bottom screw terminal of the USB-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 USB-1024HLS and possibly the computer.
Once you start running applications with the USB-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 USB-
1024HLS, is the difference between the total current requirement of the USB-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 USB-1024HLS, or you are
sinking current into the USB-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. You must
account for the supply current drawn from the USB supply for the USB-1024HLS itself, plus the current used
by each DIO line. 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 USB-1024HLS would use a significant portion of this available current, this leaves you with little or no
current for the DIO lines. For this reason, the USB-1024HLS is not supported for this configuration. 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 USB-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 may damage the USB-1024HLS because the total
amount of current is well over the maximum for the USB supply.
If you want to sink the maximum of 64 mA per DIO line, you can only do so with up to six lines. To sink
current with all 24 DIO outputs at any one time, you can sink a maximum of 18 mA per line.
3-4