# PLC Help required

Discussion in 'Homework Help' started by lomax1, Feb 19, 2014.

1. ### lomax1 Thread Starter New Member

Feb 6, 2014
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0
Hi all, I have this question which I need assistance on an assignment:

A PLC analogue input module is described as: 8-bit, 0 - 10 v dc
What is the smallest change in voltage that this input can detect (i.e. the resolution)?
What changes would need to be made to enable the input module to detect a change of at least 3mV for a 0  10v dc input?

2. ### tshuck Well-Known Member

Oct 18, 2012
3,531
675
So, how many steps can be represented by 8 bits? What happens if you divide the highest voltage this device handles by this number of steps? What are your units?

How many steps does it require before you get a step of =< 3mV. How many bits are required to represent that number of states?

3. ### shteii01 AAC Fanatic!

Feb 19, 2010
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Jul 18, 2013
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8 bits represents a resolution of 255.
So you can calculate the smallest change that 10v will represent.
Max.

5. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,388
497
256, not 255.
"Resolution limits the precision of a measurement. The higher the resolution (number of bits), the more precise the measurement. An 8-bit ADC divides the vertical range of the input amplifier into 256 discrete levels. With a vertical range of 10 V, the 8-bit ADC cannot ideally resolve voltage differences smaller than 39 mV."
From NI: http://www.ni.com/white-paper/4806/en/

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