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Voltage Analog Input - Open/Short Circuit Behaviour

Hi all,

Quick question about voltage analog inputs:

I'm using a 1746-NI8 Analog 8 Channel Input card for a SLC processor, configured for 1-5V input corresponding to 3277-16384 proportional value inputs. I'm curious as to what value I would read at the card in the event of either a short circuit or open circuit at the input terminal.

I'm thinking that for a short circuit, I'd read something like 0 Volts and hence 0 value. But what would I read for a open circuit? I would test this out, but I don't have an input card at the moment.

Thanks in advance. Ed

Reply to
Ed C
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The NI8 is capable of detecting a broken input. You choose where you want the value to go when you config the card. Each channel is configured independently. Choices are zero, upscale, or downscale.

Reply to
Ursa Major

National instruments has a web forum. Lacking access to that, I would expect to read a noise 0V on an open.

Jerry

Reply to
Jerry Avins

Jerry Avins wrote in news:e- OdnRxVT5ZWHJrZnZ2dnUVZ snipped-for-privacy@rcn.net:

A short is predictable, but an open is far from predictable. Think about how a sample and hold works on an individual card. If there's no discharge path, you might even get an artifactual copy of what was sampled on the previous (valid) channel.

Generally, you should short analog in channels that you're not using.

Reply to
Scott Seidman

A short is not always a good idea. A resistor to ground may be better. The resistor can go to either rail as well, so +/- MAX is also possible. These connections can sometimes be made internally.

Jerry

Reply to
Jerry Avins

Bits 6 & 7 in the channel configuration word specify the open circuit behavior, the input value can be set to go upscale, downscale or to zero.

Bit 12 in the input status word goes high when an open input is detected on that channel.

The module has no way to detect a shorted input.

Reply to
BFoelsch

Thank you all for the input. You have been most helpful. Cheers. Ed

Reply to
Ed C

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