High Treason's Museum

Systems

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High Treason's Museum - Systems - Sophia \ Quick Overview

Overview

Sophia powers on without incident

Spurred on by the reasonable success of the IBM 5150, IBM released its successor in 1983, the 5160, later dubbed the 'XT'. This was mostly an incremental upgrade, using the same 4.77MHz 8088 and offering little beyond an additional three expansion slots - the 5150 had only five, where the 5160 had eight, though the eighth slot is 'special' - not really, but it is wired slightly differently. Slot 8 is essentially considered an external bus and cards using it must be able to generate a select signal. Not all cards can, though some have compatibility switches for it. In any case, the system was otherwise quite successful, even if a little disappointing, which didn't go unnoticed.

Naturally, like any other successful system, it wasn't long before people figured out how to copy it. In principal this is quite easy, because the PC used off-the-shelf chips with no exclusivity contract, meaning that anyone could buy those parts and assemble a PC. Indeed, the CPU itself was already being cloned, albeit under license, as IBM had (sensibly) insisted on having 'second sources' in case the product sold well and Intel couldn't fulfil the quantity of an order. Otherwise the system was largely glue logic, available from countless manufacturers. The real obstacle was the BIOS, as IBM themselves owned the rights to this and didn't really like the idea of other companies using it. This didn't stop clone makers, who in some cases did just steal and modify the IBM BIOS, but before long it was reverse-engineered and third party versions became available. Companies such as AMI and Award would offer their own BIOS, which a motherboard manufacturer could then buy a license for and implement it in their PC Compatible, all without having to worry about legal issues and the wrath of IBM. It goes without saying that the results varied greatly from one BIOS to another and across different hardware. Some systems were more compatible than others.

With the 5160 successfully cloned, it really didn't take long at all until people looked to improve upon the design. For one thing, the 8088 was available in versions specified to run at higher clocks than the meagre 4.77MHz version. Also the IBM motherboards required minor alterations to take the full 640K of RAM which was a problem easily eliminated. Mostly leveraging cheaper Asian manufacturing and, certainly, lower quality control measures, clone systems rapidly began appearing which ran at 8MHz or higher, could be expanded up to 640K of DRAM right out of the box and even had power supplies better suited to hard drive installations - a costly option that wasn't widely adopted at first. These machines would garner the moniker 'Turbo XT', thanks to a button marked 'Tubro', which allowed the user to select between the higher clock speed for performance and the original 4.77MHz for compatibility.

At their lower price point, the Turbo XT machines proved quite popular and even helped getting PCs into the home. The PC platform still had a very long way to mature, but the building blocks were now there and the Compatible market would rapidly expand, their compatibility and features would improve and, eventually, surpass IBM in sales and capabilities. Indeed, IBM only really held their technological lead for one more generation, the 286, which was again rapidly outperformed by Compatible systems, which then went on to beat IBM to using the later 386. Love it or hate it, this was generally good for everyone, except perhaps IBM, who had created a market only to increasingly have the carpet ripped out from under them as time went on.

Our system here is a rather late one, being manufactured in 1988, though examples exist from the 1990s. Despite being barely a year away from the 486's eventual release - the 386 being three years old already - despite VGA being a year old and despite the 286 looking reasonably priced by this year, the Turbo XT remained a popular choice for its low price. It usually came equipped with a monochrome video card - normally at least Hercules compatible - or at best a CGA card. Some systems still came with less than 640K of RAM. Rarely the user would pay the extra money for a tiny hard drive. Hey, most software was still written to work from floppies on the 8088 at this time, because of the sheer numbers of such systems, and would remain this way into the early 1990s. It was no speed demon, but the Turbo XT was still a perfectly capable machine for the next few years. Such a machine would have been many people's first experience with a PC. Perhaps it wasn't the best one, but it was good enough and it mostly got the job done.

Beware, if you plan to pick up an XT machine: Whilst using the same DIN5 connector, the keyboard interface is slightly different and you will need a compatible keyboard.

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High Treason's Museum - Systems - Sophia / Specs

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System Performance

TopBench is the only benchmark on record for this machine - not because it cannot complete other tests, but because there was little point. Like most Turbo XT systems, this one scores identically to just about every other 8MHz 8088 computer with a CGA video system.

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Pictures

Almost all Turbo XT cases look like this. Big white boxes with space for either one or two Double Height 5.25" bays. The single bay suggests that this is from a time after the AT appeared. The Turbo Switch looks very much like an afterthought and doubles as the indicator for the turbo, with an LED inside.

The rear of the system is quite busy. With very little built into the motherboard, where there would be no room anyway, you had to use expansion cards for just about everything. It's small wonder, then, that the extra three slots were welcome.

The inside of the system is rather cluttered. Thankfully combo cards like the Penta-IO leverage the parallel nature of the ISA but to fit more features on one PCB. This card offers 2x Serial, 1x Parallel, 1x Joyport and a Real Time Clock for timekeeping.

Without the RTC, the system has no way to store the time when powered down. Similarly the system doesn't store its configuration in a form of RAM like later computers and is, instead, set up using dipswitches - there's not much to configure, just video card type, number of floppies and whether you have an FPU.

The motherboard doesn't feature a fancy LSI chipset. They did exist by the time this one was made, but just like earlier machines, everything is still out in the open, which is almost like having a working model of the PC. You can flat out see how the system works.

Early PCs didn't have 16-Bit ISA, due to the 8-Bit bus of the (otherwise 16-Bit) 8088. It wasn't even called ISA early on. This causes trouble when you want to install hard drives, as you're limited to only a few interfaces that are available for an 8-Bit bus. Most were MFM, but we settled on the less common XTA.

XTA was an 8-Bit version of IDE, released after the 16-Bit version. It offers automated drive detection and is easy to work with. Unfortunately it's rare enough that it may be impossible to replace something, should it break. This hard drive uses very little power, which is good because of our limited 63 Watt PSU.

They didn't just clone the PC, but its peripherals. This CGA card is very similar to the original IBM part, but uses a UMC clone of the Motorola CRTC. Otherwise it's all discrete logic driven by the system. Curiously the 6845 was designed as a companion chip for the Motorola 6800. One has to wonder.

Sometimes the system is switched over to use this EGA card when needed. Chips & Technologies were the first to clone IBM's EGA implementation and reduce it mostly to just a couple of ASICs. It's actually quite an elegant solution, but compatibility on mine is not 100%.

The RAM is made up of 36 DIPs. RAM was expensive, maybe most of the system cost, so it was better to replace individual ICs if one failed. Also this is the majority of the system's power consumption, at around 30 Watts. Swap for CMOS if you want to run colder and save power. I don't really care.

Of course the CPU is a clone, too. You can replace these with NEC V20 CPUs to gain more speed, but I think that defies the object of owning an XT. You may as well just buy a 286 at that point. If I wanted fast, I'd use a Xeon. My XT isn't there to be fast; it's there to be an XT. It does this job exceptionally well.

Be mindful that if you work on a chassis like this one, look on the underside to access two of the hard drive screw holes. Also a last note on floppies: 360K only without a ROM installed to expand floppy capabilities, or to hook the hard drive. The latter is usually included on your hard drive controller, of course.

For good measure, the system uses this AdLib clone card. Like the PC itself, there was nothing to stop anyone copying the AdLib. They didn't even have to worry about patented firmware. Not a great deal of titles use this card on the Turbo XT, but it's nice to have for those few which do.

You can click the above pictures to see full size versions in a new window

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System Achievements

Sophia holds a few records:

• Oldest design of PC I own

• Slowest Desktop PC in the collection

Ooh-laa!!!

Videos Available

Sophia's main overview video, which goes into more detail than this article ever realistically could.

Video for Windows v1.1