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  • So we took sheriff apart the other day and one of the things we saw in there was that they had this back play which

  • 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