Project /

MFM HDD Emulator

Vintage MFM hard disk drives are becoming rare. I had some success in the mid 2000's buying via eBay a few MFM HDDs for very little money, the days of people cleaning out their closets and 'just getting rid of the stuff' are over. Now, a typical 20MB or 40MB MFM HDD sells for more than $100 on eBay. Spending that kind of money on devices that are prone to fail 40 years after they were manufactured, which is what prompts the need for more, is not sensible. It would be wonderful to continue to use the old drives, in their old systems, if it were possible to have a ready supply of working replacement drives, but that is not possible. Thus, I believe that a need exists for a reliable and inexpensive replacement for these MFM HDDs that provides for the continued use and enjoyment of these old vintage computer systems.

My wish list of features for such a replacement includes:

Very inexpensive, so that utilizing multiple MFM HDD emulators is not cost prohibitive, with each emulator costing, say, less than $50.
Allows each system to contain its own dedicated emulator, eliminating the need to open up a system, extract the device, re-install in another system when interesting in running the 'other' system(s).
Have a physical form-factor that allows the emulator to be mounted in and occupy the drive bay of the drive being emulated.
As with the previous item, provide the same electrical characteristics of the replaced drive, that is, utilize the same physical cabling as the replaced drive.
Have a reasonable data storage capacity. It would be great if the emulator could serve as a replacement for a spectrum of MFM HDDs such as the Seagate ST-412 (10MB) all the way to the Maxtor XT-2190 (150MB).
Use a removable CompactFlash or SD-card, or USB thumb drive as the storage media, which would allow for quick replacement of a 'disk' by simply swapping the media device.
Perhaps prohibitive if costs are to be kept low, but it would be very useful to have the stored data appear as blocks of stored data (see flux transition discussion below) in a simple FAT16 or FAT32 filesystem.

A little research in the past couple of days has been enlightening and helps me to understand why these devices aren't already prevalent. The trouble is that the interface to between the host controller and the HDD itself is not simply digitial signaling back and forth. The control signals are that, yes, but the data that is written and read to and from the drive is not TTL, rather it is differential, ala RS-422, and it consists of the series of the raw flux transitions that are used in the magnetic medium itself. Thus the data is not just a block of bytes or bits, rather it is a continuous stream of these flux transitions, and so it is not possible just to buffer up a block of 8-bit data bytes and write it or read it from some other media. There is a lot of very low-level electrical stuff going on that requires very precise timing. This, I now understand, is a 'hard problem' (at least for most, including me) and that probably explains why such an emulator is not available from other hobbyists.

My research did turn up a couple of other emulator projects/products:

https://www.pdp8.net/mfm/mfm.shtml - This is an interesting project. Not only does the device emulate an MFM HDD, it also can be used to image, or back-up, HDDs without the use of a host computer. I applaud the designer, but this implementation is 'overkill' for what I want to accomplish. The cost if prohibitive, and requires the use of a BeagleBoneBlack (BeagleBoneGreen) single-board computer. All told, the cost of this emulator with the BeagleBone is around $200 each. Ouch!
http://www.datex-dsm.com/dtx300-mfm-emulator.html - Commercial products that are so expensive the prices aren't even listed on the product's website. I did find one page that gave a price of 1200 Euros for one device of unknown capacity! Obviously not for the hobbyist.
http://www.drem.info/ - This product is reference in several news groups/lists/fora but I could not get the website to display. Apparently this product has a max capacity of 64MB for each emulated HDD, but it may support up to four (4) disks. (I couldn't determine if that was 'at the same time' or just which drive, one of four, that was emulated at any given moment. Did get the page to load and the single unit price is $256! Still way too high!

So, there are several solutions, but it seems that there should be a means for the hobbyist to get along with a replacement emulator for a lot less money.

My thoughts on making a simple (inexpensive) MFM HDD emulator:

The flux transition data could be stored by the emulator as just a raw series of bits, where a 0 bit is at the 'low' level and the 1 bit is the 'high' level of each transition. Then the stored data is simply spewed back to the host as a stream of these bits, just like a real drive would do.
This means that the data on the emulators storage medium (CF, SD, or USB thumb drive) is not directly accessible by transferring the storage device to a PC. The PC would need some kind of software program to read out the bit-stream and convert that into a block of data.
Perhaps a very low-cost microcontroller could do the job, rather than a $55 BeagleBone Black or $35 Raspberry Pi, say, a $3 or $4 PIC part.
A first prototype version would ignore the desire to have multiple drive support, configurable drive geometry, and multiple 'partitions' on the storage media. Rather, we would start with emulating, say, an ST-225, a typical 20MB HDD, prove the concept and then expand the capabilities in a future prototype.

Some MFM HDD facts:

5 Mbps data rate to and from the HDD
3600 RPM of the platter translates into 16.67msec per track read or write