The IBM PC/AT had a troubled history with hard drives. And if you’re using one today, even if it had one of the better hard drives in it, after 35-40 years it may not work. Here’s what you need to know about IBM 5170 hard drive types to substitute newer drives for it.
The IBM 5170 defined either 14 or 22 different hard drive types, depending on the BIOS revision. You’ll get best results if you choose something that can emulate one of the existing types.
Hard drive types in the IBM 5170 BIOS
The first (pre-1985) models of the IBM PC/AT shipped with 20 megabyte hard drives. Their BIOS defined 14 different types. This was limited, but IBM didn’t see it as a problem, because they didn’t expect you to buy drives from anyone but IBM. And in the 1980s, computer makers designed computers with a shelf life of around five years. The drive sizes ranged from 10 megabytes to around 112 megabytes, which was huge in 1984. Yes, I said megabytes, not gigabytes.
The 1985 revision added eight more drive types, to support additional models. IBM used MFM-type hard drives, which used specific drive geometries. IDE-type hard drives can emulate different geometries, so you don’t have to match the IBM types exactly. But if you use a vintage MFM drive, you have more limitations. More on that in a minute.
Hard drives had their specifications printed on a sticker on the drive; The sticker contained the number of heads, cylinders, and sectors per track. To calculate the capacity in bytes, you multiplied those three numbers, then multiplied that by 512. To convert the capacity to megabytes, you divide that number by 1024, then divide by 1024 again.
Table of available drive types
|Type 15||–||–||–||–||Do not use|
|Type 16||612||4||17||0 (all)||20.3203125|
|Type 23||306||4||17||0 (all)||10.16015625|
Of course, people didn’t just buy their drives from IBM. It was much cheaper to buy a drive from a third party, even if you had to pay them to install it. Many companies made drives that matched the specs the PC/AT knew about, and they might even put the IBM drive type on the sticker. IBM did make hard drives, but frequently bought drives from other companies and had them put IBM branding on them. Of course, a Seagate drive would work fine regardless of whether you bought it from IBM or from Seagate.
When a drive didn’t exactly match, you could get away with using a different drive as long as two or three of the parameters matched and the mismatched parameter was smaller. You lost a bit of capacity, but the drive worked.
Once IDE hard drives became common, a lot of these limitations went away. IDE drives can translate different geometries, so as long as the size matched, or was slightly smaller, an IDE drive would work.
If you want to put a solid state drive in an IBM 5170 for cost and reliability purposes, you can replace the MFM controller card with an IDE adapter, and add an SD to IDE adapter. For best compatibility, use small-capacity cards. a 64 megabyte card will work when you specify type 4, and a 128 megabyte card will work as type 9.
I use solid state solutions with vintage PCs and recommend them.
Some cards and drives just don’t get along well with the 5170, though. In some cases, you can set it to drive type 1 if all else fails, and it will function as a 10 megabyte drive. That’s disappointing, but better than no drive at all. In some odd cases, even that doesn’t work. When that happens, you have one last option. Maybe two.
Replacing the BIOS
The IBM 5170 BIOS is a bit picky, and having to load setup off a floppy is incredibly inconvenient, so some IBM PC/AT owners replaced the BIOS with a third-party BIOS. Most 286 motherboards are extremely similar, so some hobbyists today will burn a copy of a clone BIOS and put it in their 5170.
Many clone makers extended the BIOS, adding more drive types. The first 23 usually matched up fairly well with IBM, but over time, PCs came to support around 46 drive types, plus a user-defined type that let the owner enter parameters manually. A third party BIOS usually supports higher capacity sizes than IBM did, and is much less picky when you use an oversized drive or card.
Another option is to add an XT IDE BIOS to the system. This doesn’t require an 8-bit XT IDE card. You can just burn the BIOS and put the BIOS in an available ROM socket, such as the boot ROM socket in a network card. You probably want one of those anyway. Then you just tell the PC/AT BIOS that it doesn’t have a drive, and let the XT IDE BIOS detect the drive at boot. This allows you to use much higher capacity cards in an AT.
Purists may prefer the XT IDE solution over replacing the BIOS. The system doesn’t look or feel as much like an IBM anymore when you put someone else’s BIOS in it. If you want convenience, that’s a good thing. If you want the vintage IBM experience, it’s not.
The controversy with early IBM PC/AT hard drives
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 AT had 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. Once IBM worked out the drive problems, the AT’s reputation recovered and of course the 286 had a long life on the market.