Analog input signal range

[Apollas]

Hello,

 

I'm studying an example project and I found one thing that it's not crystal clear to me. It's about analog inputs.

MI0 is a 10-bit 4-20mA analog input, so you have to use 204 to 1023 (820 units) for the linearization. In the net above it, it's checking whether the signal is between 190 and 1040. I believe it's checking for underflow/overflow. But why 190 and 1040 values? Why those values are used and not others?

Same situation with MI6, a 12-bit 4-20 mA analog input. 819 to 4095 (3277 units) for the linearization and 800/5020 for the underflow/overflow detection.

Maybe those values in the example are not correct. So, my question is how do you calculate analog input values for the underflow/overflow detection. In Siemens there is a table with those values (see attached), but what about Unitronics? I wasn't able to find such table.

 

Regards,

Giedrius K.

[Eyal Koren]

Hello,

 

I'm studying an example project and I found one thing that it's not crystal clear to me. It's about analog inputs.

MI0 is a 10-bit 4-20mA analog input, so you have to use 204 to 1023 (820 units) for the linearization. In the net above it, it's checking whether the signal is between 190 and 1040. I believe it's checking for underflow/overflow. But why 190 and 1040 values? Why those values are used and not others?

Same situation with MI6, a 12-bit 4-20 mA analog input. 819 to 4095 (3277 units) for the linearization and 800/5020 for the underflow/overflow detection.

Maybe those values in the example are not correct. So, my question is how do you calculate analog input values for the underflow/overflow detection. In Siemens there is a table with those values (see attached), but what about Unitronics? I wasn't able to find such table.

 

Regards,

Giedrius K.

 

Hi Apollas,

The over/under range values are determined by each product separately.

To find these values, you should look in the products specifications found in http://www.unitronics.com/support/technical-library

[Apollas]

I've already checked those pdfs and I definitely couldn't find the values that I mentioned.

The info about it is really poor in the specifications. None at all or couple of sentences about OUT OF RANGE, but nothing about the exact values or how to calculate them.

[cantcliff]

Hi Apollas,

The over/under range values are determined by each product separately.

To find these values, you should look in the products specifications found in http://www.unitronics.com/support/technical-library

 

I think his question is more along this line.

The linearization numbers are clear, as those are directly from the manual.

What isn't clear, and it's not in the two pictures he's posted, is why in the ladder above the test is for an integer value of 190 and 1040. 

On a 10 bit number, shouldn't an integer value 1024 signal out of range, you shouldn't read 1040 unless you're putting excess current into the input in which case the module should error out as too high.

@Apollas

The low end 190 should still work because the analog input actually can read the input down to zero MA, only a negative current should trigger a -1 in the reading.  190 is most likely an arbitrary sanity check.  The 1040 isn't as clear as that should be outside the modules valid ranges.

 

[Apollas]

Thanks cantcliff. The thing is that I dunno if that example I'm looking at is correct. Also, I'm new to Unitronics, so I was just wondering maybe it's a standard trick for testing an analog signal.

As far as I understood it's not a common thing to do and this example might be working properly or might not. Also, it could 191 instead of 190, it wouldn't make a difference, because it's below 204 anyway.

[cantcliff]

It's not necessarily common, but looking at the spec sheet for the V350 w/ 10 bit analog in I get this.
 

The product can read 0-20ma, or read 4-20ma. Based on some guess work from different pieces of documentation, the analog in function for 0-20 and 4-20 is the same style of read.

 

If the value is above 20ma, it reads 1024 as your analog input, so you can specifically check for 1024 for above range input. 1023 should be 20ma.

 

Where I'm not 100% clear either, and I can't test it as I don't have the hardware, is if you set the input for current, as long as you are above 0 and less than 4, I believe you still get a valid input value from the sensor attached to that analog input.  Once you go below zero the analog module returns -1 for under range.

 

Testing for 190, 191 is pretty arbitrary if my underlined section is true.  You could just as easily test for 203, 150, 135, 199, 7 or, 1. for your lower bound, the attached sensor will always provide somewhere between 4-20ma when functioning properly.

4-20ma is almost arbitrary on it's own, the main advantage is by supplying current to a sensor you always know the input is functioning, if the sensor dies or the line is cut, current drops to 0 and you can catch an error and you can send current regardless of distances.  You can buy 0-20 ma sensors, but you lose the safety/function failsafe.
 

[Apollas]

Thanks again.