Re: Write Signal on Disk II Question

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


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