Analogue or digital

[SeaBass]

Is anyone aware of any briefing documents that compare analogue fire detection and alarm systems to digital?  I’d like to get a better understanding of the pros and cons of each system.   

I’d also appreciate any personal opinions and experiences of the two types of system.

Thanks.

[Big_Fella]

It's an interesting topic, which I can't get a definitive answer to.

In the past I've been told that it's a bit of a misconception, and that all addressable system devices (e.g. detectors, manual call points etc) communicate with the control panel via digital transmission protocol.  This is to communicate with the control panel quicker, reduce communication errors and to send more data to the control panel.

Digital protocol, and digital addressable systems, I'm not even sure if this is the same thing.

Clearly I'm a little confused on this also 

[John Webb]

I note from a flyer from BREGlobal that was in the IFE/FPA journal that came in the post this morning that there is a new book "Automatic fire detection and alarm systems", BRE Ref BR 510, now on sale for £22.50 plus P+P. Haven't seen it and I can only hope it's better written than the one produced a year or so ago on places of worship. See what they say about it at www.brebookshop.com, it may be what you want.

As I understand it an addressable system has to be digital and use digital protocols for the communications between detectors/control panel/sounders etc., so one is part of the other, if you follow me.

The other important difference between analogue and digital systems is that in an analogue system it is the individual detector head which decides if there's a fire or not; in a digital system it is the control panal that usually takes that decision based on the individual readings supplied by the detector heads around the protected area. This allows prewarnings, the detection of faulty heads and other interesting factors in a more sophisticated fashion than an analogue system can achieve.

[David Rooney]

"Analogue addressable systems" - common term but probably not absolutely true with today's modern panels. I'm tempted to say all systems of this type use analogue technology within the sensors to monitor the ambient air for smoke / heat etc these analogue levels are converted to digital signals and either sent as an "interrupt" to the panel or is picked up by the panel as it polls each device on the loop.... but it's always dangerous to say "all"......!

.... Depends on the system protocol.

The panel then compares the information from the sensors to pre-programmed algorithms to decide whether the information being returned is recognisable as a fire condition or a pre alarm or a dirty detection chamber or similar.

Conventional detectors can sometimes be thought of as "intelligent" compared to analogue sensors as it is the conventional detector that makes the decision to go into fire condition as a pre engineered response to ambient conditions.

The conventional detector effectively switches a resistor across the detection line to draw a measured current from the control panel which causes it to go into fire alarm condition.

[Big_Fella]

So in summary, as I'm trying to get my head around this from the previous comments, and correct me if wrong:

Analogue addressable - detector makes the decision, either on or off / fire or no fire and the panel will act upon this.

Digital Addressable - The panel constantly analyses the data from all devices and acts upon what data is received from the devices against re-programmed patterns within the control panel (by the manufacturer).

And data on both these systems is transferred to the control panel digitally

[John Webb]

So in summary, as I'm trying to get my head around this from the previous comments, and correct me if wrong:

Analogue addressable - detector makes the decision, either on or off / fire or no fire and the panel will act upon this.

Delete the term 'addressable' as the panel will not know which detector has operated, only which particular zone has an activated detector within it - hence the requirements of BS5839 to limit zone area to minimise search times.

Digital Addressable - The panel constantly analyses the data from all devices and acts upon what data is received from the devices against re-programmed patterns within the control panel (by the manufacturer).

And data on both these systems is transferred to the control panel digitally

No - see my first comment above - it's only the 'Digital Addressable' which uses a digital transmission system. Every detector, call point and so on has an individual 'address' by which the control panel interrogates each device to find out what it's reading in terms of smoke, heat or operation. It is this ability to address each individual alarm device which gives rise to the term 'Addressable system'.

I hope, with out confusing the issue further, to explain that all detector heads are 'analogue' - those on a classical system make their own  decision to say that there is a fire when their analogue measurements exceed a pre-set level; those on an addressable system pass on their analogue measurement when addressed by the control panel.

[David Rooney]

BF..... forgetting all the engineering talk... put very simply.....

Conventional detectors - these are neither analogue nor digital. Smoke or heat gets to a certain level they switch a resistance within the detector and the panel sees this causing red zone lamps and sounder circuits to activate... and that's about it.

Just about all "analogue" systems actually use digital serial communications between the loop devices (detectors/call points/sounders/I-O units etc) and the control panel as it's far more robust to interference. The loop detectors use analogue technology to monitor their surroundings (imagine a roller coaster going up and down like a sine wave) and these various levels get given a value eg 1 - 10.

If the value is between 1-3 the panel may see this as a dirty device. 3-6 may be normal, 7 could be pre alarm and 8+ could be fire. All these levels could be changed at the control panel to vary sensitivities.

The "addressable" bit simply means each device gets assigned a number the control panel recognises as unique. You can then give each address a text message and vary the properties of individual devices and do cause and effect programming and all sorts.

To add just a little confusion, there are still some systems that use conventional heads on addressable bases !

[Wiz]

Ruby, are you actually asking about 'addressable' and 'non-addressable' systems? You have used the terms 'analogue' and 'digital' but I think you are probably referring to 'non-addressable' and 'addressable'.

I'm pretty sure that when someone talks about 'digital' fire alarm systems they are probably talking about what are commonly known as addressable systems.

But using the terms 'analogue' when describing the type of system that is not addressable can be confusing, because most (but not all) addressable systems should also properly be described as 'analogue' addressable anyway. (for reasons I will only go into if anyone is in the least interested in my long-winded ramblings!)




 

[Tom Sutton]

Ruby, are you actually asking about 'addressable' and 'non-addressable' systems? You have used the terms 'analogue' and 'digital' but I think you are probably referring to 'non-addressable' and 'addressable'.

For the lay person like me that makes it far less complicated the old conventional being the non addressable and the modern systems being the addressable/intelligent. However the other submissions take it a little step further and are very interesting. Sad old git aren't I.  

[SeaBass]

Thanks for all of the responses so far. It's interesting reading.

 I understand addressable and non addressable, but I haven't got a grasp on the pros and cons of digital and analogue systems.  Some company's promote their systems as being digital and I'm curious to know what advantages this may have over the more commonly advertised analogue systems. 

I believe that the difference between the two systems is in relation to the way in which information is transmitted around the system. Digital being in square (on /off) wave forms and analogue being in sinusoidal wave forms.   A claim that I've heard from one manufacturer is that digital signals are less prone to unwanted alarms arising from ingress of water and insects, (if you think about the fluctuations in current that these two contaminants are likely to cause then this makes some sense) but whether or not this is fact I have no idea.     

Are the digital/analogue claims of manufacturers just an advertising angle with no real substance, or are there advantages and disadvantages between the two? 

[Wiz]

I stand to be corrected, but I am not aware of any addressable systems that use analogue signals in their communication function directly between devices and control equipment. So I think there still may be some confusion.

In my understanding, you may well call the method that non-addressable systems use for determining 'normal', 'fault' and 'fire' conditions as 'analogue'. It is also true that a 'digital' value is obtained from an 'analogue' monitoring function within some addressable devices. But as far as I am concerned all addressable communication is entirely 'digital' and that trying to communicate information quickly by some sort of sinusoidal wave form would be fraught with problems.

Again, I ask if the original question was really about the benefits and disadvantages between non-addressable and addressable systems or, more probably, those  between basic addressable systems and the more advanced addressable systems that use the analogue (normally rising/falling voltage) monitoring function within a device to provide a whole series of monitored levels that are turned into a digital signal that can be easily transmitted as a series of pulses in an addressable system and are therefore commonly known as analogue addressable?

[Fishy]

Think of it like this:

Analogue to Digital (A-D) conversion is like throwing a ball up a set of stairs.  The height that the ball goes is an analogue value, infinitely variable.  However, you can assign digital values to the steps on the stairs (1, 2, 3 etc...).  If it lands on step 8, you know that the analogue value of the height of the ball is somewhere between the height of steps 8 and 9. You can therefore approximate the height of the ball to a single, digital value that can easily be transmitted over a digital network.  The smaller the steps, the more accurate your approximation is.  A-D converters do the same thing but with voltages, so the detector converts a voltage via a series of electrical measurement ‘steps’ to a digital value that can be transmitted across the network.  

Digital signals tend to be less prone to interference during transmission because the digital signal is transmitted as a series of ‘bits’ with each ‘bit’ being an on or off voltage signal (usually expressed as ‘0’ or ‘1’).  As at any one time the data signal is only telling you whether the value is on or off (not how big it is); variations in the voltage transmitted in the wire caused by electrical interference are therefore, within reason, irrelevant.  You can also get devices to send additional data that allows the receiving device to check that what it has received is the same as what was being sent, or you can send the data more than once in quick succession and check that it is the same both times.  It does mean, though, that digital data tends to be either perfect or nonsense – digital TV pictures illustrate this perfectly.  You tend to either get a perfect picture or nothing at all.

[Wiz]

Good explanation and analogy, Fishy. But do you know of any addressable fire alarm system that uses analogue signals to communicate between the cie and the devices on the wiring?

[Fishy]

Good explanation and analogy, Fishy. But do you know of any addressable fire alarm system that uses analogue signals to communicate between the cie and the devices on the wiring?

No, I don't, & I stand to be corrected but I can't see why you would want to?

[Wiz]

Good explanation and analogy, Fishy. But do you know of any addressable fire alarm system that uses analogue signals to communicate between the cie and the devices on the wiring?

No, I don't, & I stand to be corrected but I can't see why you would want to?

Nor do I, but reading between the lines of previous posts, I got the impression that some people thought that there are addressable systems that do so. I was intrigued to find out more.