David Gesswein's MFM emulator on the DEC Professional

Paul Koning paulkoning at comcast.net
Sat Jan 16 16:32:15 CST 2021

I just completed a set of tests of David's MFM emulator on my Pro 380.

Summary: everything works right.  Very impressive device.  My compliments to David for an amazing piece of engineering.


1. I built and tested it (rev C board) per the instructions and all that worked nicely.  A few minor points in the instructions, quickly clarified by David and already updated on the web page.  I used an old 2 MB BeagleBone Black (the kind that was shipped some years ago with the abandoned Angstrom distribution), that fits just fine.

2. I read the three drives I have, one RD51, two RD52 (Quantum).  All worked fine.  They were identified by the analysis tool as "Elektronika_85" which makes sense since that's a Pro clone.

3. On the RD52, the last cylinder cannot be read.  The reason is that the DEC standard formatting tool does not format the last cylinder except on the RD50.  I'm not sure why; the comments say it is "reserved for the FCT" but I don't know what that is.  In any case, ignore those errors; the OS does not use that cylinder so nothing bad happens.

4. In spite of what the XHomer documentation says, Pro disks have 16 sectors per track, not 17.  It may well be that the drive is physically capable of holding 17 sectors per track if you have an RQDXn controller, but the Pro format is definitely 16 sectors.  And 4:1 interleaved to account for the performance issues of programmed I/O rather than DMA.

5. I set up auto-start of emulation mode using one of the files created by the disk reader.  That works fine, the OS boots, identifies the drive type correctly, and runs happily.

6. I also tested creating a new empty RD52 image (i.e., 512 cylinders, 8 heads), and running a P/OS 3.0 install to that emulated disk.  That works also; as Chris Zach suggested, the installer includes a low level formatter tool and invokes that automatically when it detects it is needed.

7. Poweroff data saving works nicely.  I can watch the BBB keep running after I turn Pro power off, then after 5-10 seconds the BBB power light also goes off.

8. I looked at the extracted data files created by step 2: they are good block level data images of the disks.

A note on testing and drive copying: at first I tried to do this using a spare Pro power supply.  That does not work because that supply requires a substantial minimum current.  Even with a real drive plugged in alongside the emulator, the current draw is too small and the supply shuts down almost immediately.  Instead, I temporarily connected an old small PC-type power supply I had lying around (rated at 50 watts according to the label); that was plenty for the emulator and also good enough to power it along with the drive to be read.


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