IBM PC/XT Model 5160
On March 8, 1983, IBM released the follow up to its very successful IBM PC. The new model was called the PC/XT and it carried the model number 5160. “XT” stood for “eXtended Technology.” It offered greater expandability than the original PC, but improvements in speed and memory capacity had to wait for the next model, the 5170 PC/AT.
Overview of the IBM PC/XT
At launch time, the PC/XT retailed for $4,995 with 128KB of RAM, a single 360K floppy, and a 10 MB hard drive. A CGA video card cost $244 and the monitor cost $680. So this was not a budget system in 1983 by any stretch. If you wanted to live high on the hog and spring for the full 640K of RAM, that cost $700 additional.
Outwardly, the PC and XT look a lot alike. To tell the difference, you have to look at the badge, or look at the back and see how many expansion slots it has. An XT typically came with a single 5.25-inch floppy drive in the left bay and a hard drive in the right bay, but some people put hard drives in the right bay of a PC, or a second floppy in the right bay of an XT while replacing the hard drive with something internal. So you can’t just tell by looking at the drive bays.
Architecturally, other than losing the cassette port next to the keyboard port, the 5160 is essentially unchanged from the 5150. It still uses a 4.77 MHz Intel 8088 CPU with a 640K conventional memory limit and an 8-bit data bus and runs PC DOS. The motherboard has room for either 256K or 640K of RAM on the board, and it’s possible to modify the 256K board to take 640K.
Modifying a 256K IBM PC/XT motherboard for 640K
If you have a 256K board and want to put 640K on it, you can do so with an inexpensive, easy, and easily reversible modification.
Install a 74LS158 chip into the empty socket labeled U84. Bridge the jumper pad labeled E2 or install header pins and a jumper. It’s easiest to just solder a small wire between the two pads, and removing the wire to revert the mod is simple. E2 is near the power connector.
Then remove the 18 memory chips in banks 0 and 1, and replace them with (18) 41256 memory chips. Finally, be sure that switches 3 and 4 of switch block SW1 are set to the OFF position.
The power supply
The PC came with a 63 watt power supply, which was adequate for up to two floppy drives, but not enough to reliably power most hard drives available in the early 1980s. The conventional wisdom was that if you wanted a hard drive in a PC, you needed to replace the power supply.
The PC/XT came with a 130-watt power supply, which is wimpy by today’s standards, but was large for 1983. The XT’s 130 watts is enough to power an early 1980s hard drive and the rest of the system.
Today this is all less of a concern since XT-IDE is an option, and an XT-IDE adapter plus a compact flash card uses far less power than a 1980s hard drive did.
Expansion slots
IBM also gave the XT eight expansion slots, rather than the mere five the original PC gave you. This was important, since neither the PC nor XT had much built into its motherboard. Putting everything in expansion slots made the machine very upgradeable, but five expansion slots quickly proved not to be enough.
Why not? You needed one slot for your video card and another one for your disk controller card. Initially, both the serial and parallel port usually came on their own cards. Once you added a memory expansion card, your machine was full.
Multifunction cards, which combined the serial and parallel ports and memory expansion onto a single card, opened up a couple of slots. But if you added a hard drive, its controller chewed up one of your slots. So you were full again, or close to it.
With 8 slots in the PC/XT, it was possible to use all original IBM parts including the single-function cards, and still have room to add an Ad Lib sound card and network card. There’s an Intel card that’s easy to get working and the 3Com 3C509 can also be made to work. Not all cards work in the 8th slot in a 5160, so you have to be careful about using that slot. IBM only intended for slot 8 to be used with the IBM Asynchronous Serial Card. So if you’re using an original IBM serial card, put that card in slot 8, closest to the CPU, to avoid compatibility problems.
With the use of multifunction cards, the XT has more expandability than most people need. Not that that’s really a bad thing.
Hard drive
The XT typically came with a hard drive, either 10 or 20 megabytes depending on when you bought it. IBM’s OEMs for the PC/XT were Seagate and Miniscribe. When the XT didn’t initially meet IBM’s sales expectations, the cancelled drive orders hurt Miniscribe’s bottom line. It didn’t initially seem to matter, but off the books, Miniscribe was reeling and went into a tailspin.
Rarity
The 5160 PC/XT isn’t rare. But it sold 2.1 million units compared to 3.3 million PC 5150s. My educated guess is that about 600,000 more Revision B 5150s exist than 5160s. And yet, any 5150 typically carries a price premium over a 5160. That’s something I don’t really get. Sure, I get the collectible aspect of an early 5150, especially pristine examples. It’s the premium attached to the later, more common Rev B 5150 I don’t get.
If there’s a perception that 5160s survived in greater numbers, that’s not a difficult theory to test. We would simply need to collect a random sample. Observing Ebay sales would be reasonably random enough. Collecting 385 unique serial numbers from Ebay sales would be enough to be 95 percent confident that the proportion of the observed population is within 5 percent of the total surviving population. It’s the same methodology that insurance companies and political consultants use.
Based on the number of sold listings on Ebay right now, I think it would take 12-15 months of observing to collect a suitable sample.
The 5160 is actually the better machine and I think it’s the better bargain. It has more expansion slots and a bigger power supply, so you can expand it further than you can expand any 5150, especially when using period-correct parts.
Clones
This kind of goes without saying, but the PC/XT was very widely cloned. Some XT clones look very much like a true blue PC/XT, with a similarly sized case and 8 slots. Somewhat later clones maintained the same size but changed the styling to look a little more like an AT. A steady supply of off-the-shelf parts manufactured overseas made small scale manufacturing of XT-class machines very easy and inexpensive. That was how Dell got started. There were dozens of companies in every major city doing similar things.
I get flooded with questions about random obscure manufacturers of XT clones. Unless they were in St. Louis, where I lived at the time, I don’t know anything about them. Even if they were in St. Louis, I probably don’t know much about them. Because they were clones built from commonly available interchangeable parts, there’s almost nothing to distinguish them.
Later brand-name XT clones took advantage of miniaturization to move the essentials onto the motherboard and shrink the machine’s size by about 1/3 while providing the same capability.
From a usability perspective, clones can be more desirable because they ran at faster speeds like 7.16 MHz or 9.54 MHz.
IBM discontinued the PC/XT in April 1987, but XT clones remained on the market beyond that date. For someone wanting to run DOS apps in the late 1980s, an XT clone provided a very affordable way to do so since many office supply stores sold them for less than $700, often bundled with a monochrome monitor and dot matrix printer. When the public lost interest in XT-class systems it lost interest very rapidly, but it was the release of Microsoft Windows 3.0 in 1990 that was the tipping point.
David Farquhar is a computer security professional, entrepreneur, and author. He has written professionally about computers since 1991, so he was writing about retro computers when they were still new. He has been working in IT professionally since 1994 and has specialized in vulnerability management since 2013. He holds Security+ and CISSP certifications. Today he blogs five times a week, mostly about retro computers and retro gaming covering the time period from 1975 to 2000.