Change to SCSI Flex Disk Pin 2 Definition

[Gene Milligan]

* From the SCSI Reflector (scsi at symbios.com), posted by:
* Gene Milligan <Gene_Milligan at notes.seagate.com>
*
The flexible disk page parameters for control and reporting of flexible disk 
device parameters in SCSI-2 and SBC failed to define Pin 2. 

X3T10 has authorized Digital, which was the only company to respond the prior 
call for flexible disk page parameters for control information, to provide a 
decision on which of the three alternatives below to address the remaining 
deficiency in the FDD page parameters:

 a) Make the page obsolete.

 b) The FDD I/O standard X3.80 contains the following definition for Pin 2. 

  8-Inch Cartridge Drive Signals: HEAD CURRENT SWITCH / ACTIVE READ FILTER
  5.25-Inch Cartridge Drive Signals: ACTIVE READ FILTER

   HEAD CURRENT SWITCH / ACTIVE READ FILTER. (To the drive; head current 
switch, mandatory, 8-inch only; active read filter, optional, 8-inch and 
5.25-inch.) This interface signal is used for two different function, depending 
on whether the drive is in the write or read mode. This signal should be in a 
stable condition before attempting to read or write.

   HEAD CURRENT SWITCH - Write Mode. The signal used as head current switch is 
mandatory on 8-inch drives only. When this interface signal is true (low) and 
the disk drive is selected, the lower value of the write current is selected 
for a write operation on tracks 43 (decimal) through 76 (decimal). When this 
interface signal is false (high) and the disk drive is selected, the higher 
value of the write current is selected for a write operation on tracks 0 
(decimal) through 42 (decimal).

   ACTIVE READ FILTER - Read Mode. The signal used as active read filter is 
optional on 8-inch and 5.25-inch drives. When this interface signal is true 
(low) active and the disk drive is selected in a read operation, the read 
signal is passed through an active filter network to form composite read data 
on tracks 60 (decimal) through 76 (decimal). When this interface signal is 
false (high) inactive and the disk drive is selected in a read operation, the 
read signal is passed directly to form composite read data. The use of this 
signal eliminates the need for write precompensation. When reading a flexible 
disk cartridge that has been recorded using write precompensation, this signal 
should remain in the false (high) inactive state. 

  Neither X3.80 nor its international version 9315 has ever been revised to 
show the specifics of the 3.5-inch application.

  SCSI-2 and SBC have a four bit Pin 2 field without any further definition of 
its usage. Based upon the other pin definitions one of the bits would be 
polarity. However there is no clue as to what the other values should be. 

 c) The Pin 2 field definition and field value will be made vendor specific.

 If you have a definitive input to assist the decision, please state the input 
in a response to the SCSI reflector by 1/27/97. Charles Monia of Digital will 
forward the decision to the technical editor of SBC by 1/31/97.

Gene Milligan
SBC Technical Editor
*
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