DETECTING POWER CYCLING OR COMPUTER CRASH
REFERENCE > ADVANCED TECHNIQUES >

Detecting Loss of Power with i4xx/i100 (not i60x)

In some cases, power will be interrupted to a computer or external equipment, and one must take into consideration the affects. Two cases are discussed below.

1. instruNet Device (i4xx or i100) power cycles off/on (e.g. power outage, blown power fuse) while the computer continues to operate.
   
   External instruNet device and internal computer power are physically separate, and this scenarios involves device power Down while computer power is Up. Here, the device (i100 or i4xx) analog and digital outputs will reset, and no message will be sent to the controlling software. To detect this, the controlling software on the host computer can set an instruNet output (analog or digital) to a known voltage (e.g. 4V) at the beginning of the experiment, read it back continuously during the experiment, and if it jumps to its reset value, then it knows the device power has cycled, and that its outputs have all moved to their reset positions. In which case, the controller software can act accordingly. If working with i4xx cards and a programming language (e.g. C, BASIC), then one can call the Get_Module_Information() function and look at the 'this_module_exists_and_is_useable' parameter. If it is false, then the hardware must have cycled power (or never power up calibrated in the first place due to internal damage).
2. Computer power cycles off/on or computer crashes.
   
   External instruNet device and internal computer power are physically separate, and this scenarios involves device power Up while computer power is Down. In this case, the device (e.g. i100 or i4xx) will continue to output voltages at the same level as done before the computer went down. This assumes the device power remains on while the computer goes down due to running on a different power source.

Watchdog

If you want your Device Under Test (DUT) to go into a reset phase at this point, you need a Watchdog, which is a periodic signal from the computer to the DUT that says "I'm alive and well", and if that periodic signal stops, then the DUT detects this and enters its reset phase. This Watchdog can be implemented with the instruNet defvice putting out a periodic signal (e.g. output pulse), yet the DUT needs additional circuitry to interpret this. What circuitry? The watchdog pulse can go through a diode to charge a capacitor that is being drained through a resistor (peak detector circuit), and the output of this can be fed into an analog comparator that detects the state when the capacitor has not been charged recently (i.e. watchdog signal not present). This is then used to reset the DUT.

UPS

To reduce the probability of a power outage at the i4xx and/or computer, one can add an uninterruptible power supply (UPS), yet note that these provide power for tens of minutes or a few hours in the typical power outage case. They can be helpful since they give the computer time to detect the outage and act accordingly, before the UPS power runs out.