Pinout for SED9421

Tony Duell ard at
Fri Nov 25 18:32:10 CST 2005

> On 11/24/2005 at 8:45 PM ard at wrote:
> (taking a break between putting the turkey in the oven and making the
> cranberry sauce)
> >Are you sure the SED9421 is not a complete data seperator? The PCB I am 
> >looking at (from an Epson PF10 drive unit) doesn't seem to have much else 
> >around the 765 disk controller chip.
> I was trying to interpret the Japanese comments--it's entirely possible
> that the writer was simply drawing a parallel to the 9420, not saying that
> the 9421 used  the 9420.

Looking at am Amstrad manual, it would appear that the pins on the 9421 
aee a subset of those on the 9420 (and I don't think that's because you 
link them together). And looking at the PF10 board, it must be the 
complete data sepearator, there simply isn't enough logic on the board 

The PF10 is, of course, a 3.5" 40 cylinder drive for the PX8. Inside 
there's one PCB on top of the chassis half of which appears to be a 
ridiculously complicated PSU, the rest of which consists of :

A 63A03X microcnotroller + 2764 EPROM + 6116 RAM
A 765 disk controller + 9421 data seperator
Some RS232 buffers (there are 2 8 pin mini-DIN serial ports on the back, 
one to link to the PX8, the other to link to another PF10. From what I 
can determine, they work at RS232 levels)
An HA16642 read/write circuit
A couple of motor driver chips to drive the head stepper
Some glue logic. A few gates, a decoder (which seems to be an address 
decoder). No obvious sequential stuff.

There is no seperate logic board on the drive mechanism. The heads, track 
0 sensor and stepper plug into this logic board. There's a 6 pin plug 
that fits in to a socket on the spindle motor PCB under the chassis (the 
signals are helpfully labelled on that PCB, and are ground, +5V, Motor 
On, Index, Write Protect, and Disk Inserted).

> Has anyone noticed a significant performance difference between the PLL
> type data separators (e.g. the WD1691/2143 combo) and the plain-Jane WD9216
> digital ones?  I suspect that the PLL type is more tolerant of drive speed
> variations, but that hardly matters all that much with modern drives.  I

Nothing to speak of. I've certainly never had real problems that were 
traced to, say, a 9216 design. But then I've even had few problems with 
that infernal built-in sepeator of the 1771...


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