The Old Computer Cases 286/386/486 and AT Standard and AT power Supply.

Remember the Old 286/386/486 Cases. Meet the old AT Standard computer power supply. 

    In the evolution of computers, the most remembered and acclaimed period in Brazil and perhaps in the world, will be the 80s/90s. The classic XT/AT 286/386/486 really were an unforgettable standard. So striking that some characteristics jump out at you, clearly identifying these computers. "I had an AT 286 at the time of 20Mhz with MS-DOS 4 and Windows 3.11, at that time computing was very technical and put the user in contact with the most intelligent electronic equipment ever manufactured in the century"  (JMJG)

The Video Bellow are in Portuguese Language Only (but in the future will be translated)

After watching the video, let's talk about some additional technical features.

The XT standard case was usually "lying horizontally" exactly like the first one, highlighting a super monitor on top of it, and its huge 5 1/4 units. The AT 286/386/486 standard case, on the other hand, was usually standing up (vertical) and had some striking features on its front panel.

1. The first one was the inclusion of a 3 1/2 disk drive, more modern and with greater capacity.
2. A window with a small LCD numeric display, which indicated the clock speed. At that time, computers needed to highlight their performance through speed. The AT 286 had speeds of 10 MHz to 36 MHz or more, and this was extended to the AT 486 standard, which reached an incredible 100 MHZ! Those who upgraded to the Pentium 1 and took advantage of the case could now change the numbers to 166 MHz or more!
3. It had the famous TURBO button next to the Reset button! This button is unique and only present in the X86 family (Pentium no longer had this function on the button, it was via physical configuration).
4. Some models had a key (with padlock) to protect the keyboard against unauthorized use (yes, at the time, the MS-DOS operating system and others did not have the password feature). This way, the owner of the computer could "lock" the keyboard to prevent strangers from using the computer.

    The Turbo Button was very important at the time for compatibility issues with XT (8088) processors that operated in the 5 Mhz range or less. With the AT 286 standard, processors had speeds ranging from 6 Mhz but the latest models already had 10 Mhz or 20 Mhz! This was a problem, because many Games and Applications of the XT standard were inherited to the AT and could be run normally, the problem is that developers used the processor clock (speed) as the basis for the application speed. So if a game for example ran "normally" on a 5 Mhz processor when it was executed on a 10 Mhz computer for example, it would become completely "unplayable - unplayable" because it would run very fast at twice the speed!

    To solve this in a simple way, the TURBO button manually toggled the processor speed, interfering in the clock base between the full value (Full Speed ​​Turbo) and exactly HALF (Slow Half Speed). Thus the user of an AT with 10 Mhz could run that game (XT 5 Mhz) just by turning off the TURBO, this would change the original clock of the machine to half, exactly 5 Mhz and the game would be fully playable again at normal speed! To alert the user to the speed of the machine, there was an LCD display that indicated the number in Mhz, if the Turbo button was pressed the Full speed in Mhz would be displayed, if it was turned off, the speed would be displayed exactly at half!

    That's what that unique LCD display on standard AT X86 cases was for!

As an option, there could also be an LED indicating TURBO ON/OFF or the LED with the writing HI/LOW, just to confirm the status of the Turbo button! It was a retention button, press to turn on and press again to turn off, it was not a pulse button like it is today in modern cabinets!

The standard AT Power Supplies were actually the first computer switching power supplies that existed, already used in the first PCs of the XT line.  In appearance they are very similar to the current power supplies (popular low performance), kind of square and with a classic fan for cooling.

But the AT power supply has big differences. 

• The On/Off button of the computer is of the retention switch type, press it once to turn it on, press it again to turn it off. It is quite large and has strong terminals, because in fact raw electrical energy flows through it. All this because the AT source must be turned on/off manually, that is, there is no way for it to turn on or off by itself! With this, the main difference from an AT source is that the on/off button on the computer panel is connected directly to the AT source and not to the motherboard as is currently done in the ATX standard!

• 
  The power connectors on the motherboard and the AT power supply are in the two-by-two 6-wire standard, known as P8 and P9. 

They are very similar, the technician can easily get confused and connect them incorrectly, burning the motherboard/power supply, causing huge damage! 

The right way to connect is to always leave the black ones together in the middle!

Therefore, although switching power supplies have greater advantages in size/weight/current than old linear power supplies, they still do not have any interactivity features with the operating system (automatically turn off, ACPI, suspend, hibernate, etc.).

I hope you enjoyed our trip to the past. The classics NEVER die!

See you next time!

JMJG

Electronic Eng