plc course work

hi I was hoping I could get some help new to plc part of my course im doing the question im struggling with is as follows

Describe the term resolution and relationship between analogue inputs, outputs and word length

any help would be great thankyou

Hi,

Read this: http://en.wikipedia.org/wiki/Analog-to-digital_converter

I am still kind of new to PLC programming my self but maybe I can answer your question in some detail and some other members could add some more to this. Resolution to my understanding is the precision in which a PLC can control a analog value such as a voltage from 0-10vdc utilizing memory registers. If you have a PLC with 32 k memory then you would have 1024 bits/1k * 32 = 32768 and this would be the memory value that that processor would be able to use as the highest resolution value. Their is some formulas for scaling of analog inputs and outputs that you can search on.

It's a specification of the input card itself and not the PLC memory.

Analogue inputs will have as part of their specification a numerical value for the extents of its range e.g. 4mA=0, 20mA=32768. So in that not very real world case the resolution would be 2048/mA. You should dig through some PLC manufacturer catalogues for real world examples, there can be a bit more to it e.g. engineering units/raw values etc. however fundamentally that's what is going on.

As amberman, resolution is just how many bits per unit number of bits can vary widely, units are what you are using amps voltage, you then take that ratio and compare it to the external load your sensor is measuring, temperature for example 1 mA = x degrees then you will have bits per degree

in above example 2048 counts per mA, 31.25 deg per mA = 65.536 counts per degree (realizing there are no fractional counts, you would either use 65 or 66, noting there will be an error regardless which number is used if you need precision you can write routines to compensate)

In my college we I did mostly microprocessors and FPGA we did all the scaling and converting manually in code there was no conversion tool so I got used to doing this quite often

to answer your question a little more simplistically, word length determines available resolution of analog inputs and outputs.

Word length of devices converting the digital signal to analog (or vice versa) tend to have the smaller word length so normally those devices will determine the resolution

In other words resolution is the relationship between word length and Analog I/O

analogue inputs vary from 1 bit (yes high/low), via 8,10,12,16 up to 24 bits resolution.
a word is 16 bits, a double word is 32 bits.
In INT (16 Bits) you will need to be careful how you do calculations to prevent of loosing precision

agree with DLWEBER

When using the term word length it refers to how big the "number" in question is

The word length of an INT is 16 bits, at least from my experience that is how it is referred, especially when you talk languages where the "number" can be any number of bits up to where you run out of hardware resources.

You don't see it that much in PLC's because they come with dedicated registers of certain sizes, but there is types of programming where you have to define how many bits is in every variable you make(I have worked with numbers with a word length of over 100 bits)

hi all

I would firstly just like to say such a huge thank you for all these reply so kind the information has helped me loads

thanks guys

kind regards mo