SCSI on the small and cheap

Tony Duell ard at p850ug1.demon.co.uk
Tue Dec 7 13:50:42 CST 2010


> the problem is MFM is a disk level interface and you have to do revere MFM
> to get the data and then interpret the byte stricns for sector numbers 
> and also look
> at the step and direction for cylinder, the return side would require 
> data in MFM format

This has been discussed here many times before...

I wonder if you could do somethjing akin to the DiskFerret and simply 
record the transistions (sampled suitable fast) of the write data signal 
and replay those samples when readoing. OK, it would be wasteful of the 
flash memory space, but as many ST412 hard drives were around 20MBytes, 
but modern flash memory cards are a few Gbytes, I don't think that would 
be a major problem

You would ahve to continually replay the bitstream, of course, to 
simulate the rotating disk (whcih outputs the same track again and again 
until you chenage head/cylinder), but I think it's possible.

> and other responses like TRK000.  EDSI is really the same thing  faster 
> with a bit more
> control intelligence.

I think ESDI is sltightly easier is that the data sepaator is in the 
drive and you have clock/data lines on the interface conenctor. Thus you 
can simply record the data line beased on the write clock and replay it 
based on the read clock, you don't have to sample at several times the 
data rate. This would use the flash memroy more efficiently.

> 
> In the end to do that is a lot of complication where a MFM drive 
> directly would be easier
> or to drop on the bus and grab all the control signals before they go to 
> the MFM controller.

That depends on the machine. It;s OK to make a new controller card when 
the disk controller is a seaprate PCB (as in most PCs, Unibus and Qbus 
machines, etc). It's a lot harder (and less desirable historically) when 
the hard disk controiller is part of a larger PCB -- e.g. the EIO board 
in a PERQ. 
 
> The fact that I was grabbing ST5xx drive and ST2xx drives when they were 
> almost junk
> for free has paid over time as I have a store of drives to avoid this 
> level of pain.

I am not all convinced that an unused hard drive will 'keep' for ever...

-tony



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