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
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
So this approach works great on something like the sun
[3160] where you're going to mix and match [cards] to build a bespoke system the [works] [adjust] exactly as you want it
So you put in the ethernet cards you want the scuzzy cards the ram attention to be consistent?
But actually the same is true on the original ibm PC system. So we look at the Kit ibm PC
I've got here you had a series of five is a as they became known
Extension cards where you could put in different [cards] to build up your system?
These are the [spaces] here, and you were just literally slotting cards like this one here
which was a network card actually slightly later on so it's got a 16 bit extension, but this was the [original] card thing here and
This again have settings where you can set the address
Where you [want] to appear in the [member] now the PC is a bit more complicated [as] well as having
An address space like the motorola chips it also has an i/o address space and so you could actually things in two different ways
but the same principle
applies, and so you would then just swap these in and it will just connect up the address buffers and the data buses and the
control buses in the same way
Now the interesting thing is that there's no reason why the rest of the components couldn't have been on
Isa Cards as well the only reason they aren't is that most people are going to buy a computer like an ibm PC
Are going to buy?
one with a cPU
With ram and so on so it's just easy to put them all on the mainboard and then you have a mini backplane equivalent here
You could slaughtering the extension cards whether you wanted a monochrome graphics card or a color at the time?
Hard disk control the floppy disk controller serial port Parallel port ETc Etc
You have a same amount of flexibility the difference was if you wanted to replace the cPU card
In that [you] just slip the card out
But a better one in [turner] [to] place the cPU on here
You'd effectively have to replace the whole motherboard and it got a lot more messy, but it's a different use cases
You don't expect people to buy a computer ready built and perhaps add an extra card if they needed a network
Someone buying this knew exactly what it wanted to do and configured it to their own
Specification and when you're going to finish this ibm PC
Yeah, that's a good question when I got some time to get soldering and stop rebuilding older systems now
You're probably thinking. I just slot
My car PCI card into the computer, and it just works well the trick there is that what they're actually doing?
is that they're working out the address of where things are based on which card is an
Automatically assigning addresses and then [faq] producing away, the sort of plug-and-play
Idea that the computer can automatically configure itself for the same effective idea each card will have a unique address
And then they'll be accessed [of] course on a modern PC. Today the bus systems that are used
Look more like network connections, and they do address buses and data buses
But they're still accessed from the software point of view in the same way
could you