October 27, 1982 is the date on the IBM 5150 Revision B BIOS, making this week as good of a time as any to look over the differences between the Revision A and Revision B IBM PC 5150. What are the advantages and disadvantages of a Revision A vs Revision B IBM PC 5150? Maybe more importantly, is one of them more rare than the other?
Outwardly, you can’t tell the difference between the two machines from the front. But they have some very definite differences that can make one or the other of them much worse for you as a hobbyist. Which model IBM PC 5150 you’ll prefer really comes down to your priorities.
Outward differences between a Revision A vs Revision B IBM PC 5150
To tell the difference between a Revision A and Revision B, you have to take a look at the back. The Revision B is marked with the letter B in a circle. The Revision A has no such marking.
But if someone resprayed the case at some point, that marking may not be there anymore. In that instance, look at the screws. The Revision B has five screws in the back, or at least it has positions for five screws. It’s very common for one or two of the screws to be missing. The Revision A only has two.
I was definitely guilty of putting IBM PCs of all types back together with only two or three screws at my first job. We had a 20 oz cup from the local pizza joint full of those screws in my old work room, and I certainly put more screws in that cup than I took back out. I sure wish I knew what became of that cup. Its contents would be worth a fortune now.
A third difference is the power supply. The original 5150 power supply is black, rather than raw metal.
Advantages and disadvantages of IBM PC 5150 Revision A
To a collector, the Revision A machine is more interesting. It’s older. It’s more scarce. The Revision A 5150s are much more likely to have single-sided floppy drives. It also has the early motherboard that only took 16-64K of RAM.
And it provides a snapshot in time that not a lot of people preserved.
They didn’t get preserved in that state because they weren’t as useful. Even though the notorious and frequently taken-out-of-context quote that 640k ought to be enough for anybody referred to this very machine, it didn’t actually let you use all 640k if you installed it. Its upper limit was actually 544k due to a limitation in the early BIOS. The early BIOS also doesn’t enable the BIOS on VGA or EGA cards or hard drive controllers.
The single-sided floppy drives are also a liability. They have half the storage capacity of the later and more common double-sided drives, and the majority of PC and XT class software shipped on double-sided disks. This makes using the machine more difficult, or it limits what software you can run on it.
If you want a historical artifact that forces you to use a PC the way someone would have used it in 1981 or 1982, a Revision A 5150 is absolutely perfect. You probably won’t run into those limitations because almost nobody was pushing those limitations yet.
But if you want to see just how much later software from the second half of the 1980s or even the early ’90s will run on a 5150, a pristine Revision A 5150 is the very last thing you want. Even if you do install a double-sided drive in it, which was a common upgrade, the memory limitations will get you. Forget about running anything newer than DOS 3.3 on it because of the memory consumption. For that matter, you may want to stay with version 2.1 or older.
You can upgrade the BIOS, assuming someone didn’t do it already, but then you no longer have an original Revision A.
Advantages and disadvantages of the Revision B IBM PC 5150
From a compatibility standpoint, the Revison B 5150 is the better machine. It can take a full 640k of memory. It is much more likely to have 360k drives in it. There’s even a long shot possibility it has a 720k floppy in it. And it will run anything that doesn’t require a 286, as long as the software is compatible with whatever video card you have in it.
If you want something that looks like the original IBM PC from 1981 but acts like a 5160, a Revision B 5150 is a great hobbyist machine. It still creates a time capsule, but its time frame is less narrow.
A phantom rarity
The downside to the Revision B 5150 is they aren’t rare. But they tend to cost a lot, because you are paying for a perceived rarity that doesn’t exist. A Revision A 5150 commands a price premium over a Revision B 5150. But a Revision B 5150 carries a price premium over a 5160. That’s in spite of the 5160 being the less common of the two machines. The 5160 didn’t replace the 5150. IBM sold them side by side. Near the end, the 5160 outsold the 5150 because people wanted hard drives. But you could still get a 5150 right up until the end of the line for the original IBM PC series in April 1987.
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 if you’re using period-correct parts.
IBM sold 3.3 million 5150s and 2.1 million 5160s. We don’t know the complete breakdown of revision A to revision B, but revision B had a production run of 4 years. Revision A had a production run of closer to 18 months, ending in March 1983. And we know IBM sold about 60,000 IBM PCs in its first six months. On page 12 of the Oct 10, 1983 issue of Computerworld, I found an estimate from Hitech Software Engineering that IBM was selling 45,000 PCs per month. If we assume that number was also true from February 1982 to March 1983, that would put sales figures of Revision A at around 600,000 units total. The actual number is probably lower than that. But I can’t find a better estimate, so let’s go with it. With 3.3 million sales total, 600,000 revision As would mean about 2.7 million Revision Bs exist.
It also means about 600,000 fewer IBM PC/XT 5160s exist than Revision B 5150s.
The pragmatic view
At this point, I don’t have a 5150 of either persuasion, or a 5160. And if I encountered one in the wild, I’m not sure I could resist the temptation to bring it home with me. I have a 5170 and a Tandy 1000, so I don’t need a 5150. But that’s easy for me to say when I’m not standing right in front of one.
From a pragmatic standpoint, the best IBM PC for most of us is the one right in front of us with a price tag on it, ready to buy.
But if you have a choice, it’s natural to have a preference.
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.