David Gesswein's MFM emulator on the DEC Professional

[Paul Koning]

> On Jan 17, 2021, at 1:35 PM, Paul Koning <paulkoning at comcast.net> wrote:
> 
> 
> 
>> On Jan 16, 2021, at 5:32 PM, Paul Koning <paulkoning at comcast.net> wrote:
>> 
>> 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.
>> 
>> Details: ...
> 
> Some more details.
> 
> 1. I forgot to mention that the Pro disk controller always uses only drive select 1, so that has to be set as the primary drive select on P7, and nothing on the secondary (P8).
> 
> 2. RD50s are odd; they use pin 32, which is normally drive select 4, as an output to indicate "I am an RD50". The software looks for that signal.  So to emulate an RD50, install the jumper for drive select 4 (alongside the one for drive select 1) on P7.
> 
> 3. Since on the RD50 drive select 4 is an output, you can't plug one of those drives into the emulator either for reading or writing, since the emulator (also) drives that pin.  You'd have to open up that wire or something along those lines to avoid having a battle between two drivers.

David pointed out #3 is incorrect.  I was thinking of totem-pole drivers, but the drivers in the MFM interface are all open collector.  So the usage conflict between RD52 using "drive sel 4" as an output vs. the controller also doing that is not actually a problem at all.

Meanwhile, a new item:

4. If you have an emulated drive that doesn't have either 4 or 8 tracks, and you need to or want to format it, the P/OS formatter will do the wrong thing.  It will format whatever the last track is as if it were the 8th track, because the emulator currently treats invalid head selects as selects of the last valid track.

I haven't yet run the RT-11 formatter but from examining an old listing I have that appears to be that formatter, it should avoid this issue.  

This won't matter if you're reading an real drive into an emulator file, so long as you don't try to reformat it.  It does matter if you're initializing a new emulator file from scratch.  If so, there isn't a whole lot of reason to use such drive layouts; just pick the RD53 (67 MB) or if you need a 31 MB drive, the 8 head Quantum RD52.  Those format without trouble.

	paul