All the cards connected to so what I thought

We'd do is actually to have a look at how a backplane works and how it?

Interconnects all the different cards to form a complete computer system

And what [it] will actually show us is how any computer works?

it doesn't matter [whether] it's using a motherboard like a traditional computer or a backplane system or

Computers while this all computers up to the pentium era are connected in this sort of fashion

We see the backplane, physically what it actually looked like

diagrammatically is you've got a

Pcb and you've got various sockets on it

And these are all

interconnected

[you] also have some resistors on there and probably some capacitors to smooth the power supply and to terminate the ends of the bus

But effectively you've got a series of Connectors which the cards can slide into and everything is then interconnected?

So what sort of things are connected over those first of all if we have a look over here?

I have a descendant from one of the original acorn system cards

This is a control universal euro Beebe card, but he's is the same backplane technology as the acorn system used

So originally all you've got areas of pins and when you plug these into the backplane

They just connect up with the socket and then any data [or] anything

We sent down these wires connect you talk to any other cards, so effectively. What are you going to send over the backplane? What's basically?

the signals from your cPU

so you've got a

six [8,000] cPU here actually took it out of an old lady writer printer and coming out of this you have an

address bus and [we've] talked about this before and

You have a data bus and the data bus is bi-directional

So I'm just going [to] move the trip a bit so I can actually sort of draw another arrow

[so] we've got an address bus and we going to date with and the way the cPU works

Is a few other control signals which go over there as well?

Which we'll ignore for [now] is when every wants to talk to anything in the computer

Inputs on the address there, and that's just a number so we have something get addressed one zero one zero

One zero and it will say I want to read that address

[I] want to write that address and if it's reading it than the data sent that way if it's writing it that it writes the

[Data] out all that goes over your backplane

Effectively along with the control signals is the address bus and the data bus if you look carefully

You can see them coming off the cPU here being can directly to the pins on the backplane Connector now

This is a bit odd if you look at the original schematics for the acorn system one

You'll see that they had a whole load of buffer chips

We should make sure you've got a strong and stable signal out over the backplane bus

One of the problems when designing these sort of things is that the voltage [that] comes out of the cPU or microprocessor?

is

Struggling off the drive and motherboard when you got [two] going over a back [plane] or something

[you] probably want a buffer there to give a bit more strength to the signal so it can talk

To the other things and they would have buffers so that when they send to me back

The Data didn't get caught because he was travelling along the relatively long wide if you look at the sun system again

You [see] [we] have quite a wide backplane, so let's have a look at our backplane. So we've got two things we've got an

Address bus which [we'll] call, [Led] [Dr]

And we have a database and we'd also have an alarm which as a control bus. Which is also be there

It says whether you're reading or writing

What type of data triaxis and so on we're going to ignore that for now?

But it would be there as well, and it connects up in exactly the [same] way

So if we wanted to build up a system

With the backplane there we just sliding the cars and effectively all [you're] doing is connecting that device to

The address bus and the database so we started off with a cPU card [and] so we've got a cPU there

And that connects to the address bus and it connects down to the data bus like that

Now we also want some other things in there

We want to be able to use the system to probably some memories

so we'd [have] a ram card and again that connects to the address bus and

It connects to the data bus probably opting out from there

we had a scuzzy card in there and this again connects to the address bus and

It connects to the data bus and of course because it's scuzzy it's got a connection to the hard disk

as well as a separate connection and the files and cpus concerned only see the scuzzy card and then the discovery card talks to the

Hardest so he sort of talks indirectly and you could put whatever else you wanted on there

So we could have an ethernet card now cause there's nothing to stop the cards having other things there if we look to the cPU

Card that we had before in the sun system it had the cPU on there

But it also had four meg of ram, and it also had an ethernet controller and some serial port so the card can be

multifunction, but in effect

As far as we're concerned here

We're treating is one thing now they're connected to the address bus and the connected to the data bus so how do they communicate?

Well, they all communicate over the data, but that's where the data sent

But you need to make sure that only one thing is communicating at a time otherwise

You'll get a garbled signal on there, and you wouldn't know what's happening

so the way [that] the cPU does that is it make sure everything has a unique address and

So one of the things that you need to do is make sure that

The ram is only accessed when you've got one address on there

Because he's only accessed when you've got another address on there, and so on depending [on] what you need to do

So how did you get it about doing that so we look at a real computer system here now this wasn't a back playing basic

Computer this was the Atari, St

Book laptop, so it's also [to] combine onto one motherboard

But the principles [are] [same] the difference between this and the sun is it with the sun you could swap the components around as you

Please to choose it with a laptop having a huge cards and there would perhaps be a little cumbersome

So everything was built on to the one motherboard so [the] first thing we see when we [look] at it

Is that you dress [buses]?

and the date was in some cases the [dress] buttons are different sizes the cPU like these six [8,000] we have here has a

24-bit address, [but] now because it's a 16-bit cPU or present the 16-bit data

But you only have 23 address bus pins that you need to take out of it, so it's labeled

23 here so we've got 23 bits worth of address space. We can access that

16,777,216 bytes worth of Memory Or Hardware

Whatever you want to put in there now as we go through we can see the other things that are connected so for example

We have a system rom, but this only uses 18 bits worth of address space

It's got 18 bits the meeting control on the machine and you use two bits worth as did the keyboard controller the sound chip

They only been drawn on the address connection there, but probably only uses two bits worth there

if I remember right destroy it in

slow [one] [-] Charlie went [bus] if they didn't draw their diagrams properly this one was an interrupt and

Multifunctional device, it used five bits. That's 32 bytes

Worth of address space and so on so how does it work? Well the basic principle? Is that you decide?

What you want to go where in your computer so [with] something like the AtAri?

they decided that if this was the memory will use hexadecimals like 16 meg's and

zero they said right the first four megabytes are going to be [ram] so any address between zero and

What's four megabytes is hex? I think that's four zero zero zero zero zero

I may have an extra zero in there, but it's about that, sort of value

So that's going to be round to any address in this range. We're going to say is around what they also said is that anything?

Roughly and for the [chart] [rams] in there

I am waving my hand so all the hardware things are going to go up there

and then you had your rom trip which started at e 0 0 0 0 0

And that's got the operating system in so what we can see from this is different addresses

Different numbers [of] the location refer to different things so as far as the cPUs concerned

We put an address on the address bus [and] we're talking to something else [we] put [a] low address

We're talking to [ram] could put a high address

We're talking to hardware and so on so what the computer has to do in fact what?

every single card has to do in a batplane system like this is look at the address on the address bus and decide whether it's

An address that's accessing it, whatever it provides

Or whether it's an address for something else which case it ignores it and how to do that one way

I [think] it's really helpful to see how this works is to look at phone number

So in the uk we might have a phone number like this

Which we familiar to anyone of a certain age in the uk anyone have an older age maybe expect me to written

Oh one eight one one eight zero five five hello great good morning to us

Just reading one of the morning papers anyway doesn't matter there's the collection of numbers in this case

They [are] 1 2 3 [4] 5 6 [7] [8] [9] digits on they can vary computer addresses are a fixed number of binary digits

the analogy works

But this number isn't just a series of digits. It's made up of two parts

We have an area code or std code as a known in the uk

And then the phone number within that exchange in this case

We can split it like this and this is

s

Td code and this is the rest of the number and what happens when you dial in to swap shop is you dan that number?

The first two digit tells the exchange that you want to connect to the London exchange

And then Coax the London exchange with other exchanges the rest of the number

To connect you to the BBC and hopefully you get through to speak to no [Redmon's] but for the younger uk audience obviously

As TV colors have [changed] a lot since?

1980 [yeah], so yeah, this is what they were up in the 1980 now Sean rightly points out. We can change this

It doesn't have to be this and actually in the late 80s. They would change so they were actually

[O8] One and so you had to

Change the exchanges to recognise that prefix and then they could use the rest of it

But this bit didn't change and it's exactly the same with our computer system

So [our] computer system has an address [say] for the rom chip and in this case

It's going to access in binary [one] [one] [one] do it which is he and hexadecimal and then some [more] digits?

And we'll just fill these zeros

[kept] running out of space it's a this is why we use hexadecimal when we're doing computer systems because binary numbers take up

But as far as the computer is concerned

It works exactly the same principle as the phone numbers and the phone number

This is the bit [that] refers to something in the London exchange and [it's] refers to the London exchange

The computer works in the same way this bit

Refers saying this means it's wrong and this fit tells you where it is in the wrong now

I can change this by just making the computer look for a different number here

and so what you need to do is have some sort of logic and because these are zeros or one they can be treated as

digital logic to or False and Saunders true

when that number is one when that number is one when that number is one and that number is zero or not one and

You add them all together, and if that's true. You know you're talking to

wrong

Now if you've got a different number here say, we're talking to hardware

One one one one sort of like that the [longview] numbers are unique you can

Access different things so all you have in your system

Connecting everything together is on each of these cards you have a bit of logic

which will look at part of the address on the address bus a

Certain number of bits and say is this the address that's been assigned to me for this card

Yes, it is therefore. The address and the data on the address bus are from my car

odd

If not

Been there for one of the other cars and ignores it so they'll be a bit of logic there that says is this the address

I've been assigned if it is I'm going to handle it

if it isn't

Then I'm not going to handle it something else can deal with it and [if] we have [a] look at one of the cards

If we pull a card out [of] the system

So this is from a [sundry] 160 server

And it's got an ethernet connection on the back

[those] [are] [some] free wide euro cards if you look the connectors are the same as the one we had earlier

But if you look these three rows of pins in there, and two rows here, [but] if you look here

you've got a series of

chips and still given dust and a series of dip switches that enable you to program in a

Specific address and that will then set where littles here in the computer's memory and so you can then pop that back into the system

And it would allow you to talk

to the second Ethernet card