No, PS/2 DBA disks are not ESDI

One of the most common questions I get from people is "Can I put the ESDI-Emu in my PS/2 Models 50/55/70?"

Sadly, the answer is no. Why not? Because the PS/2 Models 50/55/70 doesn't use ESDI disk drives.

IBM wanted a physically small disk drive that wouldn't require a separate controller. What they came up with is a unit that fits what we would call a 3.5", half-height disk drive with an integrated controller that implements everything including the system bus interface. That makes it even more integrated than the later IDE drives, which still require a separate system bus adapter.

This was very advanced stuff for its time. It's very much like a MicroChannel Architecture version of a modern M.2 PCIe solid-state card. What it is not is anything remotely related to ESDI.

Here's an image of a Microscience 3.5" ESDI disk drive:

And here's an image of a PS/2 DBA disk drive's electronics PCB:

Notice the difference in connectors? The ESDI drive has two edge connectors; the wider one is for the daisy-chained control bus, while the smaller one is the point-to-point data cable. The PS/2 DBA drive has a single edge connector that takes up the full width of the board.

At first you might think the difference is just pin-out. But the ESDI interface is a clear descendant of the ST-506 (MFM) disk interface, which requires a controller card plugged into the computer's internal bus. The PS/2 DBA interface is a subset of the IBM Micro Channel Architecture computer bus and plugs directly into the computer's internal bus. 

Let's just say they're very, very different interfaces.

Honestly, I'm not sure where the term "DBA-ESDI" comes from. At this point, IBM still used the same term for disk data storage they'd used for decades: DASD, for "Direct Access Storage Device". The IBM technical reference for the PS/2 Model 70 the connection of this device as "Direct Bus Attachment", which probably explains the "DBA" part. But I've found no references to "ESDI" in any of the IBM PS/2 documentation I've searched through.

My guess is that whomever is behind the Ardent Tool of Capitalism site assumed it was an ESDI drive (or derived from one) and came up with the name "DBA-ESDI". And it stuck.

Given the number of people asking for a replacement for the PS/2 DBA drive, I may look into making one at some point. I've started restoring a PS/2 Model 70 for that project. But first I need to get the ESDI-Emu fielded.