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Cable lengths
SCSI cables = 18 feet
Printer cables: serial = 25 feet
printer cables: parallel = 10 feet
Coaxial cables (x.base.2 or x.base.5) = 185 meters
Twisted pair (UTP/STP) = 100 metes
Fibre optic (x.base.FL) = 2000m (practically - with commercial grade)
IRQ's, I/O addresses and DMA's
NMI = non maskable interrupt - used for parity checks
0 = system timer - used for synchronising operations
1 = keyboard
2 = cascaded to 9, practically used for video card
3 = COM 2 and 4
4 = COM 1 and 3
5 = LPT2, usually used by sound card
6 = Floppy drive controller
7 = LPT1 - printer port
8 = real time clock - used for synchronising utilities and date/time
9 = re-directed to 2 - if IRQ2 is used leave alone
10 = available
11 = available
12 = mouse
13 = math co-processor, used for floating point calculations
14 = primary hard drive
15 = available
0 = system timer
1 = used by 8 and 16 bit sound cards
2 = used by floppy drive
3 = used by 16 bit sound card in conjunction with 1
1F0 = Hard drive 16 bit 320 = Hard drive 8 bit
3F8 = COM1
2F8 = COM2
3E8 = COM3
2E8 = COM4
378 = LPT1
278 = LPT2
Register = 16 bit
Data bus = 8 bit
Address register = 20 bit
RAM 'seen' by CPU = 1M
Speed = 5/10 MHz
Socket = 40 pin DIP
Register = 16 bit
Data bus = 16 bit
Address bus = 24 bit
RAM seen by CPU = 16M
Speed = 6/10/12 Mhz
Socket = 68 pin PGA/PLCC
Data bus = 16 bit
Address bus = 24 bit
RAM seen = 16M
Speed = 16/20/25/33 MHz
Socket = 100 PIN PGA
Data bus = 32 bit
Address bus = 32 bit
RAM seen = 4G
Speed = 16/20/25/33 MHz
Socket = 100 pin PGA
Data bus = 32 bit
Address bus = 32 bit
RAM seen = 4G
Speed = 16/20/25/33 MHz
Socket = 100 pin PGA
The only real difference between the SX and DX was that 'intel' were 'clever', they had decided to include a math co-processor into the chip design - not a bad idea, but risky - the process was subject to a high fail rate - and those processors whose math co's failed were marketed as SX - they would normally have been rejected and thrown away!
Data bus = 32 bit
Address = 32 bit
RAM seen = 4G
Speed = 25/33/50 MHz
Socket = 168 or 208 pin PGA/SQFP
Data bus = 64 bit
Address bus = 32 bit
RAM seen = 4G
Speed = 60 - 200 MHz
Socket = 'socket 7' 296 pin PGA format
Data bus = 64 bit
Address bus = 32 bit
RAM seen = 4G
Speed = 233 - 266 MHz
Socket = 'socket 8' 387 pin PGA format
Data bus = 64 bit
Address bus = 32 bit
RAM seen = 4G
Speed = 233 - 450 MHz
Socket = SEC
Has on board cache (level2) to speed up processor, MMX technology - 57 instruction set for video,audio and graphic data manipulation
Data bus = 64 bit
Address bus = 32 bit
RAM seen = 4G
Speed = 450 - ? MHz (Who cares - after 500 you get dizzy !)
Socket = SEC
This processor has it all, mega on board cache, instruction sets for graphics,etc as well as multi-tasking abilities - I don't really have any more other than to say go to the Intel siteby clicking here!
OK, here's a tip - I've seen a lot of questions asking about the 'work area' of a computer - it refers to memory..
Depending on the make/model computer you will encounter different RAM
RAM stands for - 'random-access-memory' and loses contents by the removal of power.
There are different types of RAM, SDRAM, DRAM, dual-ported RAM, etc - all with their own strong/weak points
SRAM - static RAM
DRAM - dynamic RAM
Dual ported RAM - found in video/sound cards
SIMMs - single in-line memory module
DIMMs - dual in-line memory modules
On most mother-boards you will see provision for both of these memory chips, the SIMM slots are normally white and are quite short, they have a metallic clip on each side of the holder. To install these chips you must tilt them at a 45` angle, slide the contacts in and tilt upright.
DIMMs are another story, they are a higher advancement in memory size - and because of this need more contacts to the mother board, this is done by doubling the height of the SIMM slot and putting in another row of contacts in a double-decker format. To install these chips insert them vertically and push down - remember to support the board !
In an 8 bit 8088 you will need an 8 bit SIMM to fill one memory bank, in a 64 bit P2/3 you will need 64 bit SIMMS to fill the bank - you can mix-and-match ie. 2x 72 pin, 32 bit SIMMS ( 32x2=64 bit)
Just to clear up SIMMS - or confuse you completley !!!
8 bit SIMM = 8 data bits + 1 parity bit = 30 pin, won't fit on a Pentium !
16 bit SIMM = 30 pin
32 bit SIMM = 72 pin
64 bit SIMM = 168 pin
ROM stands for 'read-only-memory' . This memory is non-volitile - that means that it retains its contents when power is removed. You will find this type in the BIOS chips and in most cards needing a fixed instruction set.
As with RAM there are different types of ROM
PROM - programmable read only memory
EPROM - erasable programmable ROM
EEPROM - electrically erasible programmable ROM
UVEPROM - ultra-violet erasable programmable ROM
The most common are UVEPROM sometimes just called EPROM, you will recognise them by the glass window on the top of the chip, this should be covered by a sticker to prevent any stray light from entering the chip. To erase the chip you need a UV box, basically a light-proof container with a high power ultra-violet lamp inside.
The box is light-proof because UV rays are bad news to your eyes !
To program the chip you need a special chip programmer - there are cards available for a computer that allow for the installation of one of these programmers on your PC.
EEPROM - these are the latest-and-best of the lot, heard of "flash BIOS", well this is a EEPROM. The chip can be erased and programmed by using standard voltages - this makes it a lot easier because you don't need special equipment to re-program the darn things - all you need is a program made by the BIOS manufacturer and that's it. To get the BIOS programs for common PCs go to driverguide.com
OK, there is always a question on monitors
Know them well - it also helps in practical aspects of the job.... OK, not much but a bit !
A monitor is a type of TV, in fact a TV has most of what's inside of a monitor....
I will start with the power supply
There are actually a number of voltages floating around in a monitor, from 5 volts to +/- 20 000 volts - my advice is take off your anti-static bracelet no, really - getting bolted by a screen is not fun even if you are un-earthed - I don't want to know what it's like with a strap on !!!
The power supply uses the 5 and 12 volt supplies for the video processing, etc
What the monitor actually does is seperate the video input into slices or horozontal lines each that fills the screen from the left to right hand side - the monitor you are looking at has lots and lots of 'pixels' all over the screen - here's a practical test - lick your finger and put a drop of spit on the monitor screen, see those color blobs? they are pixels
Now, after the logics have split the screen into slices it must put them onto the screen
This is where the high voltage comes in....
A long time ago budding electronic nerds discovered/invented the 'vacume tube' - basically they found that if they made a light bulb with two metal plates inside and a high enough voltage is applied to a plate compared to another plate in close proximity, electrons left the negative plate and 'flew' through the vacume to the positive plate in a narrow beam. Now days we know how this happens, and what happens - they had no clue! - just a bit of Newton here - the law of relitivity as seen by me...
An object with mass cannot travel at the speed of light E=MxCxC - as you approach the speed of light your mass increases exponentially ie. 1,2,4,16,256,etc - why am I mentioning this ? Here is a practical example - the electrons travelling toward you right now are increasing in mass by about 300% and are travelling at roughly 75% of the speed of light (300 000 000 meters per second!)
If they had more mass to start off with, well, needless to say, you wouldn't want to be where you are now !
Ok, back to theory of operation, a high voltage is applied to the screen - if you look inside a TV or monitor you will see a lead going onto the monitor screen at the top right corner - don't touch ! this is the highly positive voltage which attracts the electrons from the 'gun' - the thing at the back that glows, more about this thing later.. all around the inside of the screen is a powder that is stimulated by electrons - that in turn has a color attached to it, either red,blue or green
By simulating these colors in the correct amounts the color spectrum is achieved by mixing.
Now for the 'gun'
This is the 'heart' of the screen, somewhere on the board is a timing and sequencing circut, these circuts control the beam of electrons travelling to the screen by making them travel in a pattern similar to how you would write on a page, from the top left corner to the right top corner, down one row, and from left to right - this is called the 'fly-back'
This beam of electrons is 'modulated'(varied) by another circut on the board to stimulate the correct pixels and thus light them up to the correct degree - and thus form a picture.
OK, practically again, a customer who knows-too-much sets his refresh rate ultra high - too high for his monitor - what happens? pretty blobs all over and nothing you can do but to restart in safe mode and correct the settings - what is happening ? Here goes, the timing circut that determines the speed that the beam of electrons must sweep from left to right can't keep up so it only gets as much as it can on one sweep and carries the rest of the sweep to the next row down - OK, here's an example, I want to write down the word
Floccynaussynihillipillyfication
but someone will hit the enter button every one second of me typing
Here goes ...
Floccynaussynihill
ipillyfication
See ? Now they push it every 1/2 second!
Floccynau
ssynihill
ipillyf
ication
Get the picture ?
The first monitors were called MDA's or monochrome display adaptors, they used to weigh a ton, and looked shit!
Graphics were limited to those that you can use with your keyboard ie. []_I/\|
These were digital monitors and are basically only found on jumble sales and museums
These monitors operated on a digital platform, that is, the data sent to them was is the form of 1's and 0's
They had four data bits, red, green, blue and an intensity bit - this meant that they could mix the colors and make the mix darker or lighter
They could manage to get 16 colors out of this mix. In graphics mode they managed a magnificent 160x200 resolution, and text 640x200.
This monitor used an intensity bit for each color giving a pallete of 64 colors, unfortunatly technological problems meant that they could only show 16 at once - which wasn't much good!
The resolution was increased to 640x350 in graphics and 720x350 in text.
Now the fun began - the boffins realised that digital was severely limited and analog was the way to go !!!!
Instead of having the red pixel either on or off (as with the digital intensity bit) they could now control the intensity of the color throughout the range. This was a good thing because now they could get 256 colors at any given time from a virtually unlimited pallette.
The screen resolution was increased to 720x400 in text and 640x480 in graphics mode.
This was the latest development, the limitations of the VGA monitor was mostly to do with memory problems - so again the boffins decided to dedicate memory to the graphics card. This meant that a pallette of 16 million colors could be shown at once.
The resolutions also were improved to 1024x768 or 1280x1240
OK, another practical tip - if a customer comes in for a monitor upgrade please ask him what monitor he is using currently - if it is a digital monitor then don't supply him with a 21" SVGA 'cos it will be toast as soon as it's plugged in - the digital adaptor will destroy the analog circutry inside the monitor - and you will be toast too when he comes storming in !
OK, another tip, before working on a PC make a note of all the connectors on the back of the PC - when it comes to re-assembly you will thank yourself !
The typical connectors you will see are these:
D-type
These are shaped like a 'D' if you look carefully and, except for the display connector, have two rows of pins running parallel to each other.
They come in two forms male and female - male has pins and the female has sockets
these come in three sizes,
9 pin - for some joysticks and mice
15 pin HDA - for VGA and SVGA monitors
25 pin - female is for printer, male is for com port
DIN-type
These connectors are normally circular and have a row of pins arranged in a circular fashion.
DIN-5 - a five pin connector, female mounted on the PC - keyboard
MINI DIN - otherwise known as the PS2 - used for modern mice or keyboards.
Centronics-type
Ok, these are mostly used in printers but are also used in some SCSI devices, they look like a D-type that has shrunk and have plates instead of pins.
Mini-sub-D - looks like a shrunk centronics-50 but has lots of pins - these are used in SCSI 2&3 devices.
RJ-type
These are common run-of-the-mill 'phone jacks and contain up to 8 pins.
RJ-11 - used for a single phone line - you will see these on modem cards
RJ-14 - used for a double phone line and are not common
RJ-45 - used in networking, these are fatter than RJ-11/14, have 8 pins and are the most common ethernet connector.
BNC-type
These are connectors that look a bit like a light bulb connector. They are usually silver and you will find them on a network card.
These are RF connectors and have a high shielding capacity
RCA-type
OK, these are for audio, you will find examples on the back of your hi-fi they are sortta like a small doughnut !
These are sometimes found on sound cards
2.5mm stereo jack-type
These are almost always found on a sound card and look like black grommets. They have 3 contacts, one for left audio, one for right and one for earth.
Kettle plug-type
OK, who doesn't know that these are for power ?
The female(on the PC) is the mains in and the male is the monitors supply.
