IBM model 5170 - Save from scrap. Fixed problem with Power supply and floppy drive adapter (Part 2)
5In August 1984, IBM launched their 2nd generation PC - the PC/AT (Model 5170), designed around the Intel 80286 microprocessor which ran at 6 MHz. "AT" stood for "Advanced Technology".
In April 1986, IBM enhanced the standard PC/AT by increasing the 80286 from 6 MHz to 8 MHz. This new "Model 339" also got a new keyboard (the Model M) with separate numeric and cursor keys, indicator lights, and 12 function keys. A "Model 319" was identical to the 339, but only got the processor speed upgrade (not the keyboard). Both came with 512 KB RAM. At the same time IBM announced two memory expansion options - a 128 KB increase to base memory (bringing total base memory up to 640 KB), and a 512 KB Memory Module Kit which allowed software access to this additional 512 KB in the protected address space. A 2 MB Memory Module Kit was also available, and you could install up to five of these at the same time, bringing total memory up to a massive 10.5 MB.
One key advancement over the 1st generation PCs, mainly as a result of now running a true 16-bit microprocessor, was the 16-bit data bus. This meant data could be transferred to and from expansion cards twice as fast as before. Furthermore, the system's address bus was increased to 24-bit which permitted memory access up to 16 MB. Both these changes were designed to be fully backward-compatible with the IBM PC and PC/XT, so existing 8-bit expansion cards would work on the AT, as would any software.
A battery-backed real-time clock (RTC) was also incorporated on the motherboard, and 50 bytes of CMOS memory were used to store BIOS parameters during power-off. Instead of DIP switches on the motherboard, a software program could be run to setup the these BIOS parameters. Later clones would have this program built into a ROM chip, with the program executed during the Power-On Self Test (POST) by the user holding down the ESC, DEL, or F1 key. Physically, the PC/AT came with a keylock, which disabled the keyboard when the key was turned.
The PC/AT came with a 20 MB hard disk (CMI model 6426-S) and the new high-density 1.2 MB 5.25" floppy disk drive (Y-E Data model YD-380), which increased storage capacity over 3 times for floppy disk users. IBM released PC DOS 3.0 to coincide with the AT's launch to support its new features.
The power supply was also upgraded to 192 W, and was a switched-mode power supply (more efficient than linear power supplies). This new power supply could be problematic as it turned out, since it requires a high load on both its +12V line and the +5V line. For PC/ATs that shipped without the hard disk, this load simply wasn't enough, so the power supply would randomly fail to start. Because of this problem, IBM started to fit a 5 ohm, 50W sandbar resistor to the +12V line of the hard disk power connector, thus pulling typical current of a hard disk if you didn't have one. This resistor tended to draw 2.4A (28.8W) and got pretty hot!
"AT-class" became a term describing any machine which supported the BIOS functions, 16-bit expansion slots, keyboard interface, and other defining technical features of the IBM PC AT.
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