Would it be a problem if I tied "Host +5 V" to ground?

[minh716]

I want to tie a switch that is normally open from host +5 V and ground. Would this have adverse effects? Thanks

[Brian_Beal]

If you like to see smoke and replace PC components, then this would be a great idea.

[AnalogKid2DigitalMan]

No adverse effects, that is, until the switch is closed.

What are you attempting to do, and what is the 'host', a PC?

[minh716]

Basically what I have is a device that I am trying to sync with NI. When NI turns off, the device will turn off and when the NI turns on, the device would turn on. What I thought I would use is NI's "host +5 V" to tie into my device. The device gets trigger by a voltage greater than 2.5V. My other problem is that I need a switch to turn off the device. The solution I came up with is located in the picture below:




What is boxed in red is something I cannot change because it is wired internally.The thing i changed was that I added a ~1ohm resistor (something small). Will this work? Thanks for the help.

[Buechsenschuetz]

The "Host +5V" output is specified for 100mA load maximum. I do not know how or why the current is limited but I do not think that it is a good idea to operate the NI card out of spec. If you need a voltage level of more than 2.5V to trigger your device, and if you are already using a 1kOhm resistor from the trigger input to GND, connecting the "Host +5V" output to the trigger input via a 470 ohm resistor will show a voltage level of approx. 3.0V on the trigger input. If you close the switch, the maximum load on the "Host +5V" output will be a few mA, well below the maximum specification. So replacing the 1 ohm resistor (this value barely makes any sense) by a 470 ohm resistor probably will solve your problem. To avoid any problems due to faulty circuits or components, I would recommend using a 100mA fast blow fuse as close as possible to the "Host +5V" output. Anyhow, there might be some glitches/spikes on the "Host +5V" output during powering on and off your host PC, so maybe the voltage level on the trigger input will be reached for very short periods before the "Host +5V" output is stable. If this might cause any problems (on our systems it does!), you would be better off using a "true" reset controller which has some hysteresis for voltage level detection.

[Michelle_N]

Brian, thank you for all your input.

minh716,

I am assuming you are talking about a 5V output on one of our motion drives - which one are you using? Brian has given a lot of great information - the one ohm resistor is so small it is nearly negligible and you should be using something larger in its place. I need know what drive you are using so I can verify the output specs.

Brian made a great point that there will most likely be spikes in the output until the system has settled - may not be ideal for your for triggering your device.

[Buechsenschuetz]

"Brian made a great point that there will most likely be spikes in the output until the system has settled - may not be ideal for your for triggering your device." Although my name is not Brian, I think this refers to my posting. We had some problems with a drive making some irregular moves as long as the host PC (with a NI734x) is switched off. Since we are using a differential driver (AM26LS31) as interface to our drive, we tried to connect the "Host +5V" directly to the enable input of the AM26LS31. This caused some awful irregular movements of the drive during power-up, and made it even worse! We have added a reset circuit with proper voltage detection and a short time-out (DS1233, there are many similar ICs). We did not have any problems with irregular output pulses during power-down or power-up ever since. The bottom line is, if you need a proper trigger pulse generated when the host PC is switched on, you'd be MUCH better off using a circuit which can detect proper voltage level on the +5V line instead of connecting the trigger directly to the "Host +5V" output.

[Michelle_N]

Buechsenschuetz,

My apologies for the name confusion.

[minh716]

Thank you for all your responses. I had not seen irregular movements with my system yet but I think I will look into the reset circuit with voltage detection and time out.