Measurement Computing USB-DIO24H/37 - Manual (Page 13)

USB-DIO24H/37 User's Guide Functional Details
13
Caution! The +5 V pins are outputs. Do not connect to an external power supply or you may damage the
USB-DIO24H/37 and possibly the computer
When configured for input, you can use the USB-DIO24H/37 digital I/O terminals to detect the state of any
TTL level input. Refer to the schematic shown in Figure 6. If you set the switch to the +5 V input, Port A0 reads
TRUE (1). If you move the switch to GND, Port A0 reads FALSE.
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 web site at
www.mccdaq.com/signals/signals.pdf ).
USB connector
The USB connector provides +5 V power and communication. The voltage supplied through the USB connector
is system-dependent, and may be less than 5 V. No external power supply is required.
LED
The LED indicates the communication status of the USB-DIO24H/37. It uses up to 5 mA of current and cannot
be disabled. The table below explains the function of the USB-DIO24H/37 LED.
LED Illumination
LED status Description
Steady green The USB-DIO24H/37 is connected to a computer or external USB hub.
Blinks continuously Data is being transferred.
Blinks three times Initial communication is established between the USB-DIO24H/37 and the computer.
Pull-up resistors
Each digital port has an internal 47k pull-up resistor network. All digital pins are pulled up to +5 V (high logic
level) on power up and reset.
Power consumption
The maximum total output current that can be drawn from all USB-DIO24H/37 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. To
overcome this limitation, an external self-powered hub can be used to supply the proper current level required
for full output capability.
Once you start running applications with the USB-DIO24H/37, 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-
DIO24H/37, is the difference between the total current requirement of the USB-DIO24H/37 (based on the
application), and the allowed current draw of the computer platform 500 mA for desktop PCs and self-powered
hubs.