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Topic Title: Digital input / outputs vs Analogue input / output
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Created On: 09 May 2013 10:26 PM
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 09 May 2013 10:26 PM
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SKElectrical

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Digital input / outputs vs Analogue input / output
Hi can anyone tell me the difference between these two so far as BMS signalling is concerned, or point me in the direction of a webpage that can explain the two.
Thanks.
 09 May 2013 10:34 PM
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dickllewellyn

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A digital input or output is either an on or an off state, an analogue could be perhaps a temperature sensor, or a light sensor, usually either 0-10v or 1-10v where a value can be assigned to the voltage. Ie. a temperature probe might supply 5v at 30 degrees, 8v at 40 degrees, so you can set the parameters as required to control accurately.

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Richard (Dick)

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 09 May 2013 10:45 PM
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SKElectrical

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ok that would seem to make sense. So what are the advantages of digital then? I take it pulsed outputs are of the digital type, say to half open a valve.
Thanks Richard.
 10 May 2013 08:18 AM
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dickllewellyn

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You can use pulses, or a sustained contact, I don't know a huge amount on the industrial control side, but in a home automation scenario you could for example press a button to bring lights on, hold said button for dimming, double click for something else, triple click for something else etc. you can use pulses as a counting method, for example a pulse output of a CT clamp. I think with most programable relays and whatnot you can pretty much make it do what you want!

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Richard (Dick)

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 10 May 2013 08:21 AM
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dg66

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Analogue gives better control and would be used where a simple on/off,open /closed scenario isnt fit for purpose.

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Dave(not Cockburn)
 10 May 2013 09:16 PM
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SKElectrical

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how can you tell if its analogue or digital in use?
 10 May 2013 09:49 PM
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Thripster

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Read manufacturers specification sheet or stick scope on it.


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 10 May 2013 10:25 PM
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jamieblatant

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My plc career started from this book

http://www.amazon.co.uk/The-PL...trollers/dp/0134898400

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 11 May 2013 07:45 PM
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prtadko

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A DI is used to monitor the current state of an open/close switch such as a flow switch. When the water pump starts or vant fan starts the flow closes the switch and the controller DI is made. If said switch is not made when the system is required then an alarm can be generated to show system failure.

A DO is used to enable/dissable an on/off state. an example would be to energise a relay to start a motor.

DI/O's are 10vdc signals from and to the BMS controller.

AI's are variable signal inputs such as a thermister temperature sensor.

AO's are variable signal outputs to a process control variable. This could be a motor speed controller or a valve actuator. If we just concentrate on motor speeds for now, 0v=off 1-10vdc= 10%-100%.
most actuators also use this signal for the same function however some actuators may require a raise/lower modulating output which would be a timed DO pulse in one direction or the other.

The way to tell what you have got is anything on/off will be Digital.
 12 May 2013 11:48 AM
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SKElectrical

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My plc career started from this book
http://www.amazon.co.uk/The-PL...trollers/dp/0134898400


Thanks. Looks good / essential.


prtadko

A DI is used to monitor the current state of an open/close switch......


Well put thanks.
 13 May 2013 01:58 PM
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AJJewsbury

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Digital isn't necessarily just on/off - put together a number of 'digits' and you can read or set almost any value you like. I'm not sure about BMS in particular, but in general, that can either be a number of digital lines (wires, terminals) side-by-side (parallel interface) - where you can have 2 to the power of the number of lines discrete values (e.g. 256 for 8-lines), or you can send each digit one after another long a single line (serial interface) - sometimes with an extra line or two for timing.

Digital has one fundamental disadvantage - you can only represent a certain discrete number values - so if you're measuring some continually varying quantity - temperature for example, there will always be some error introduced when you convert it to a digital value (perhaps you can only have whole degrees C, or tenths of a degree, but you'll often have to make do with the nearest - rather than perfectly accurate) - sometimes called a quantitisation error.

The same feature is also digital's big advantage - as values can only certain discrete level (typically 0 and 1 at hardware level) - it's dead easy to eliminate noise and mild interference - 0V and a bit of noise is clearly still 0, 1 (perhaps represented by 12V or 5V) is still clearly a 1 even if the signal goes down to 8V (or 3V). - so as far as transmitting (or storing) data, digital is a clear winner over analogue where any noise, stray capacitance, extra resistance etc directly reduces signal quality.

- Andy.
 13 May 2013 05:36 PM
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prtadko

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Originally posted by: AJJewsbury

Digital isn't necessarily just on/off - put together a number of 'digits' and you can read or set almost any value you like. I'm not sure about BMS in particular, but in general, that can either be a number of digital lines (wires, terminals) side-by-side (parallel interface) - where you can have 2 to the power of the number of lines discrete values (e.g. 256 for 8-lines), or you can send each digit one after another long a single line (serial interface) - sometimes with an extra line or two for timing.



Digital has one fundamental disadvantage - you can only represent a certain discrete number values - so if you're measuring some continually varying quantity - temperature for example, there will always be some error introduced when you convert it to a digital value (perhaps you can only have whole degrees C, or tenths of a degree, but you'll often have to make do with the nearest - rather than perfectly accurate) - sometimes called a quantitisation error.



The same feature is also digital's big advantage - as values can only certain discrete level (typically 0 and 1 at hardware level) - it's dead easy to eliminate noise and mild interference - 0V and a bit of noise is clearly still 0, 1 (perhaps represented by 12V or 5V) is still clearly a 1 even if the signal goes down to 8V (or 3V). - so as far as transmitting (or storing) data, digital is a clear winner over analogue where any noise, stray capacitance, extra resistance etc directly reduces signal quality.



- Andy.


Classic example of why i don't often post on this fourm anymore. A long-winded, overly complicated answer by an ''engineer'' to who really knows very little of the subject matter. Even going as far to say ''I'm not sure about BMS''....But here is my best attempt to give you some total b****x that will blow your mind.

The origanl question was regarding I/O's for a BMS application.
 13 May 2013 07:02 PM
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OMS

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Classic example of why i don't often post on this fourm anymore. A long-winded, overly complicated answer by an ''engineer'' to who really knows very little of the subject matter. Even going as far to say ''I'm not sure about BMS''....But here is my best attempt to give you some total b****x that will blow your mind.


Easy !!

When you know everything about everything, then get your toys out the pram is my advice.

OMS

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 13 May 2013 07:45 PM
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prtadko

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I am not so arrogant as to assume i know much of anything so the whole toy flinging bit is wasted on me. thanks anyway. If someone asks a question, i think it is only right that said question be answered. with that in mind, why try to answer if you dont know? all you do is just confuse the situation and the correct answer gets lost in all the jibber jabber. there are lots of people who seem to see it as their duty to reply to every thread seemingly just because. thats fine as long as the post is contributry. however when it is just utter tosh (even from a senior member) the point becomes lost and so does the entier reason for the fourm in the first place.
 13 May 2013 07:51 PM
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OMS

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I think you missed the point of what Andy had to say

OMS

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 13 May 2013 07:54 PM
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prtadko

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I think Andy missed the point of the origanl question.
 13 May 2013 07:59 PM
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rocknroll

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Handbags at dawn!!!

regards

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 13 May 2013 08:30 PM
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OMS

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Originally posted by: prtadko

I think Andy missed the point of the origanl question.


Which was answered in the second post ? - so we have no requirement to go beyond that ?

Regards

OMS

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 13 May 2013 08:31 PM
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OMS

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Originally posted by: rocknroll

Handbags at dawn!!!

regards


LoL - and saucers of milk all round

regards

OMS

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 13 May 2013 08:44 PM
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prtadko

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Originally posted by: OMS

Originally posted by: prtadko



I think Andy missed the point of the origanl question.




Which was answered in the second post ? - so we have no requirement to go beyond that ?



Regards



OMS


The question was indeed answered in the second post. however the requirement to go beyond that was not mine. The OP required further explanation as seen by the next question.

I only posted one reply. I just took exception to the post thereafter and the techno babble therein.
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