Floppy drive interfacing

Jules Richardson julesrichardsonuk at yahoo.co.uk
Sun Apr 8 16:41:47 CDT 2007

Philip Pemberton wrote:
> Hi guys,
>   I've just dug my not-even-a-tenth-finished floppy disc raw-reader out 
> of the cupboard, the plan being to finish it off.

That's weird. That's three of us separately suddenly fiddling with this again 
(personally I suddenly stumbled across enough SRAM the other day for a raw, 
over-sampled track buffer, which rekindled my interest in actually Getting 
Something Done).

>   - All I/O lines from the floppy raised to +5V via two 4k7 resistor packs

Hmmm, the SA-400 manual says that the lines should be terminated by 150 ohms 
to +5V on the last drive in the chain. Output line 'true' from the drive is 
logical zero, 0.0V to 0.4V, 48mA maximum. Output line 'false' is logical one, 
+2.5V to +5.25V, 250uA maximum.

Doubtless the manual's available in electronic form somewhere on the 'net - 
probably on Bitsavers.

> I can step the head in and out fine, and the motor seems to be spinning 
> (the drive's activity light is on at least). What I can't get is any 
> form of output from the drive - even /INDEX is stuck floating high 
> (FWICT it should pulse once for every revolution of the disc, i.e. about 
> once per half-second or so).

300RPM would be a 5Hz pulse rate, 360RPM would be 6Hz, so a bit more than that.



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