Last Updated on October 30, 2021 by Dave Farquhar
The IBM 5170 PC/AT is a very popular retro PC, for several reasons. It was the last of the open architecture IBM PCs. And from 1984 to about mid-1987, it was the fastest and most expensive IBM PC. It wasn’t the fastest PC in its category. Compaq took care of that. But if you wanted true blue IBM, this was the machine. It was also controversial, both in its heyday and even today. Let’s talk about why.
Technical specifications
Officially, IBM marketed the 5170 as the IBM Personal Computer AT. The “AT” stood for Advanced Technology. In all fairness, for 1984 it was pretty advanced. Unlike the original IBM PC, it was a fully 16-bit architecture and could theoretically use up to 16 megabytes of RAM, an amount that seemed like science fiction in 1984. Its 6 MHz clock speed was an incremental improvement over the PC and XT’s 4.77 MHz, but the 286 CPU was more efficient, so it was more than twice as fast as the original IBM PC. How much faster was always something people argued about, because the benchmarks of the time tended to be a bit artificial. IBM later boosted the clock speed to 8 MHz, but the design made it easy for clone makers to top its performance without harming compatibility.
The 5170 came with 256K or 512K of RAM standard and a 1.2 MB high density 5.25″ floppy drive for storage. The 512KB model included a 20 megabyte hard drive and serial/parallel card as well. The 256K model was very bare bones.
But the 5170 represents a cautionary tale. It represents the golden era of IBM’s dominance but it was also the beginning of the end.
The controversy of the IBM 5170 PC/AT
The IBM 5170 was good, but IBM didn’t want it to be too good. The 286 was Intel’s fastest chip at the time in 1984, but it ran at 8 MHz. IBM ran it 25% slower, at 6 MHz, and deliberately slowed down its memory access. Unlike the PC and XT, the Intel 286 could run Unix. That meant a PC/AT could theoretically compete with an IBM minicomputer. And if you were going to buy a PC/AT and use it as an inexpensive Unix machine, IBM wanted you to wish you’d bought that IBM minicomputer instead.
But that wasn’t all. It soon turned out that the 1.2 MB floppy drive wasn’t fully forward and backward compatible with the 360K floppy drives in other IBM PCs. The 1.2 MB drive had no problems reading 360K disks. But if you formatted a 360K floppy in the 1.2 MB drive, it didn’t always work in a 360K drive. If you wrote to the disk with a 360K drive and then wrote to it with a 1.2 MB drive, the 360K drive usually couldn’t read the disk. Soon IBM started offering a secondary 360K floppy for swapping data with earlier IBM PCs. This drive was called the YD-580. The 1.2MB model was the YD-380. The YD-580 had a molded-in dot opposite the activity LED to indicate it was a 360K drive.
But the biggest problem was the reliability.
The Computer Memories International problem
The first IBM PC/ATs shipped with 20 megabyte hard drives manufactured by Computer Memories International. The CMI 6426 quickly proved unreliable, and tarnished the 5170’s reputation. CMI struggled to provide the drive in the quantities IBM needed, and the poor quality control eventually led to IBM replacing the drive with the similar Seagate ST-4026. The disk controller was designed by Western Digital, who hadn’t yet started making hard drives.
If you somehow run across a CMI 6426, I recommend you hang onto it, even if it doesn’t work. Which it probably won’t. There can’t be very many of those left.
But it wasn’t immediately clear that the hard drive was the problem. As a result, the AT picked up a reputation for being unreliable, and until the problem was fully traced to the drives, it took the 80286 CPU’s reputation down with it. AT clones appeared pretty quickly, but there was some speculation that they wouldn’t be any better than IBM’s AT.
Once IBM worked out the drive problems, the AT’s reputation recovered and of course the 286 had a long life on the market.
Purity of essence
The controversy today is what constitutes a “proper” IBM 5170. When Youtuber Adrian Black rebuilt a 5170, commenters skewered him for everything ranging from replacing the BIOS to installing a 3.5″ floppy drive.
He admitted that replacing the BIOS made it less original and expressed regret, but he also had issues with error messages on boot that wouldn’t go away unless he replaced the IBM BIOS with something more universally compatible.
And while not the most common upgrade, people did these swaps in the 80s and 90s too. People didn’t want weird errors on boot. People wanted newer disk drives. Someone who was willing to pay $2800 for an IBM AT when it was new was willing to pay $100 to make a 601 error go away, or to extend its useful life. That’s why MR BIOS existed.
Is his 5170’s purity, or lack of it, a tragedy? There were people who said installing a 3.5″ floppy in the 5170 disturbed its purity, stating that nobody used 3.5″ drives in a 286, 5170 or otherwise. That’s false, and most 286 clone systems sold after the release of the IBM PS/2 came with 3.5″ drives.
The 5170’s appeal was that it came in a big case with lots of expansion slots and room for upgrades. Not everyone took advantage of those slots and drive bays, but many did.
Is there any such thing as a typical IBM 5170?
Yes, most photos of 5170s have dual 5.25″ drives, a 1.2MB and a 360K drive. This was for backward compatibility, as the 360K drives in XT-class machines couldn’t reliably read a 360K disk after both 360K and 1.2M drives wrote to it. It was super tricky to get a disk to work in both. So it was much easier to put both drives in AT-class machines.
That was the typical IBM PC/AT 5170 from 1984 to 1986 or so.
But IBM sold the 5170 into 1988. Later 5170s shipped with a single drive and a blank faceplate in the second drive bay. What was the second drive bay for? Whatever you wanted. And many people did put 3.5″ drives in that bay. It was usually a 720K drive because it was easier to get those working, but many 5170s did end up with a secondary 720K drive. I encountered many 5170s in the field early in my career, and every 5170 I recall working on did have a 3.5″ drive of some sort in it.
The second question is memory expansion. Some of them stayed at 512K their entire service lives. But many of them got expansion cards to bring them up over 1 MB. The expansion cards were very common. Not everyone had one. But many did.
I also find the statements that everyone overclocked their 5170 curious. I know some people did. But every 6 MHz AT I ever personally encountered ran at its stock 6 MHz.
I would argue there’s no such thing as a typical IBM 5170. These machines had long service lives and people did what they needed to do to keep them usable. But a 5170 without an upgraded BIOS, MR BIOS or otherwise, was a hassle. Y2K gave us a good excuse to get rid of most of them.
What is the ultimate IBM 5170?
In the end, the ultimate IBM 5170 depends on what you want to do with it. If you want a fully period-correct IBM 5170, then one with as many original IBM parts as possible that shipped together would probably constitute the ultimate 5170. Unfortunately the earlier 5170s are less usable than the later ones. And mixing parts from the different generations can be problematic. People who worked at IBM in the late 80s and early 90s joked that even IBM computers weren’t 100% IBM compatible.
Yes, there’s something about a 1985-era IBM 5170 with original IBM 1.2MB and 360KB floppy drives, the IBM 5154 EGA monitor, and the original Model F keyboard. It’s iconic. As a display piece or a showoff piece, it’s hard to beat. But it’s not an ideal machine from a usability standpoint. Many DOS games that support EGA need a 256K EGA card, and IBM’s original card came with 64K. And after 1987, it wasn’t long before software started shipping on 3.5-inch disks.
An IBM 5170 with a 3.5-inch disk drive and a VGA video card and monitor is quite a bit more usable. And I encountered a lot of 5170s in the early 90s that had been upgraded in exactly that way. It’s how IBM intended the machine to be used. They intended for it to be upgraded and expanded. If you want to use the machine, those two upgrades, along with a network card, make it more useful. Arguably a Model M keyboard is more useful too. And later 5170s did ship with Model Ms. A Model M intended for a 5170 looks just like the PS/2 version except for the square IBM logo in the corner.
Now if you have room to keep multiple PCs set up, a 386 or 486 is much better for running VGA-era software. But if you want to keep the 5170 set up all the time and want to use it for multiple purposes, there’s nothing wrong with having an upgraded one around.
The right IBM 5170 for you
Ultimately, if you want to play the IBM 5170 game, how you play it is your decision. They aren’t the easiest machines to come by in any state, so I recommend you start with whatever you have to work with. I’ve had my 5170 since the 90s, when it was essentially junk. Somehow I picked up a YD-380 drive at some point, but my 5170 came without a motherboard. I had a Pentium board in it for years, but installed a 286 clone board in it a couple of years back. I’ve considered picking up a genuine IBM 5170 board, but those tend to come in waves and right now those boards are hard to come by and very expensive. If I ever find a YD-580 at a good price, of course I’ll grab it. The same for any other IBM cards that would have originally gone in one.
In many ways, my Compaq 486 is a more fun machine. But I do enjoy the 5170.
David Farquhar is a computer security professional, entrepreneur, and author. He started his career as a part-time computer technician in 1994, worked his way up to system administrator by 1997, and has specialized in vulnerability management since 2013. He invests in real estate on the side and his hobbies include O gauge trains, baseball cards, and retro computers and video games. A University of Missouri graduate, he holds CISSP and Security+ certifications. He lives in St. Louis with his family.
Although the PC/AT was originally sold with the Model F keyboard, a modified version of the original IBM PC keyboard with one extra key (Sys Rq) and a new bidirectional control protocol, they were later sold with the Model M keyboard that was introduced in 1985. For me that would be the iconic keyboard to use with an AT, not the Model F.