Re: Write Signal on Disk II Question

[Mike Willegal <mike@willegal.net>]

I'm not sure if I entirely understand your question, but I'll give it a 
try.  When reading the disk, the controller derives it's timing from the 
changing state of the fields.  If you wrote eight ones and simply left 
the field in a constant state,  the controller would have a hard time 
determining where one bit ended and another one started and the timing 
would most likely be hopelessly out of whack after a just a few bits. 
This is why there are requirements in each code word for limiting 
adjacent zeros (which do not change the field) and manditory ones (which 
do change the field).


Regards,
Mike Willegal

Mike Willegal wrote:
> The book "understanding the apple II" has an excellent description of 
> the Disk II interface.  It has been converted to PDF and can be found at 
> a couple of locations online.
>
> Regards,
> Mike Willegal
>
> Bryan Parkoff wrote:
>
> >     I study Logic Sequence ROM.  It does not make sense when I look at 
> > Write Sequence table.  I decide to store $FF on data bus before data 
> > bus.  Load switch transfers $FF from data bus to data register.  Shift 
> > switch continues the state sequence to shift left one bit into QA like 
> > as MSB.  QA is always set for all 8 bits.  The Write Signal toggles 
> > between two loops while QA is set.  It looks like this below.
> >
> > QA   Data Register   Write Signal
> > 0    11111111   0   Load $FF
> > 1    11111110   1   Shift 1st bit
> > 1    11111100   0   Shift 2nd bit
> > 1    11111000   1   Shift 3rd bit
> > 1    11110000   0   Shift 4th bit
> > 1    11100000   1   Shift 5th bit
> > 1    11000000   0   Shift 6th bit
> > 1    10000000   1   Shift 7th bit
> > 1    00000000   0   Shift 8th bit
> >
> >     QA is ideal to show digital bit stream.  Write Signal is analog 
> > bit stream.  It controls the current in the coil of the read/write 
> > head.  Also, it has second head like as erase head..  The high/low 
> > state of the Write Signal determines the direction of the magnetic 
> > field set up in the read/write head.  Please explain more about the 
> > field reversal and the lack of field reversal why Write Signal is 
> > required to toggle while QA is always set for all 8 digital bits.
> >     I may be curious to see if I can simulate mangetic field to show 
> > field reversal or lack of field reversal.  It shows the same thing 
> > like Read Pulse.
> >
> > Bryan Parkoff