Why Does the Counter Source Sink 120mA When the Computer is Powered Off? Hardware: Legacy DAQ Devices>>Multifunction DAQ>>AT-MIO-16XE-10
Problem: I have an E Series MIO device with a TTL signal connected to the counter's SOURCE input. When I switch off the computer, I measure a sink current of approximately 120mA on the source of the counter. When I disconnect the power cable to the PC, I do not have this problem. How can I limit this behavior? Solution: The counter lines go directly to the DAQ-STC counter chip which has protection diodes connected to Vcc and GND to keep signals within limits. When power is off, Vcc is 0V and you just have a diode to Vcc. To keep from damaging the card or the computer, the input voltage should be kept under 0.5V when power is removed from the E Series board. You can use a resistor in series with the input signal to limit current sourced by your external hardware. To compute the necessary resistance use V/I = R and keep I (current) below 10mA. For example (5 volts)/(10 mA) = (500 ohm resistance). Realize you do not have to do this for all signals, only signals that are configured for input and then only those whose drivers are capable of delivering more than 10 mA. Anything more than 10 mA can potentially damage the DAQ-STC. Related Links: Attachments:
Report Date: 05/28/1998 Last Updated: 02/20/2004 Document ID: 19REGR7O |
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
