1734 Point I/O Analog range configuration

radfahrer

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Does anyone know why Rockwell defaults the Point I/O analog modules to 16383 (Hi) and 3277 (Low) for the analog scaling in the card configuration?

Reading the manual, it appears you can use a full 16 bit signed integer, so I'm curious is there is any reason to not use the full range of -32768 to 32767 to get maximum resolution?

I'm wondering if maybe the internal A/D converter is less than 16 bits, so using a larger range doesn't actually increase the resolution?

Any insight appreciated...

-Greg
 
The 1734 Analog Point I/O is "box configured" for 4-20 mA operation and a nimble deployment hence the present defaults; if your application requires it, using a different scaling will provide better granularity than the default one.
 
dmargineau,

Do you know where they got those preset defaults from? Does it match other AB cards? Is it based on a 15 bit A/D which does 0 - 21 mA full range and they have it for 4-20?
Those default numbers just seem kind of random to me.
 
I have never thought about it this way; however, I think your presumption is correct.
Just some quick math...for 4-20 mA (Max./Min = 5) thus 5 * 3277= 16385 which is 16383 + 0.02%...close enough...:D
What's interesting is that 32766/2 = 16383 so thay have chosen half of the "maximum" granularity for a default...Who knows?!...
 
I was looking through some of the docs and it looks like those numbers might be some legacy from SLC or PLC-5 days.
From a functional point of view, if you have a range of 16000 counts you get 0.001 mA per count, with their range of 3277-16383, it's a bit less precise than that but they're still getting <0.002 mA per count, which is probably more precise than most devices connected could perceive anyway or that could be measured by a decent loop calibrator, so in the end it's not a major issue.
I guess I'll just stick with the defaults to keep it easy.

-G
 
Look at the specs for the analog modules on AB's website.

Example: For a 1734-IE2C, the resolution is 16 bits over 21ma which is set by the cards ability to read 0.32 uA/cnt. It only needs 16 bits to display this. You can scale the card to be -32768 to 32767, but you will just be doubling the 16 bits - not gaining any better resolution.
 
Does anyone know why Rockwell defaults the Point I/O analog modules to 16383 (Hi) and 3277 (Low) for the analog scaling in the card configuration?

... snip ....

Any insight appreciated...

-Greg

I was just looking for these numbers because the AB manual only states the data format is a 16-bit integer and it does 0.32 [uA]/count.
But it seems that the default is actually a 3277 ... 16383 range.

So I rather go with that rather than setting a 'special' scaling/configuration that can lead to surprises for other engineers familiar with AB modules.

- I think this default range is inherited from older controllers.
Siemens has something similar (100% = 27648 counts).

I also think they are deliberately not using the full integer 'count' range so the user application can detect out of range situations.

Also it's not a good idea that 20 mA / 100% is close to the maximum integer value (+32767) because an increase of just one count will make the integer go fully negative (-32768) a.k.a. integer overflow.
Same thing for going the other way 'round (subtracting 1 from -32768 gives you +32767).
 
I typically scale my inputs 4000 to 20000 for current inputs/outputs. This allows me to monitor the actual current value for troubleshooting purposes. I also use an AOI that I have created with built-in alm setpoints/latching logic, etc. If I require an analog signal that requires high precision, I tend to specify devices with ethernet capability to avoid feedback issues.
 

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