11.1.2 Floppy Signals

Short description of all Floppy Signals

The pin-outs to the Shugart interface, it may help, you found under Floppy.

Pin 2: Head Load

 
A low signal on this line sets the read/write head on the diskette. This feature is designed to protect the disk because the hear rubs on the disk only when it is actually going to access it. Unfortunately, this signal is not available of the ST becuase the WD1772 controller does not have this connection available. This line is often connected to 'Motor on' however.

 

Pin 3: Ground

 
All odd-numbered lines through 33 are tied to ground. This ground conection is used for operation as well as shielding.

 

Pin 4: In Use

 
This signal tells the drive that it is connected and is used. It is not available on the Atari.

 

Pin 6: Drive Select 3

 
A low level on this line means that drive three is being selected. Only the drive which is assigned as drive three by a wire jumper in the drive reacts to commands, and all other remain neutral. This signal is not used on the Atari because a maximum of two floppies can be connected (0 and 1, or A and B).

 

Pin 8: Index

 
The drive uses this line to send a low pulse upon each rotation of the disk. This signal tells the cotnroller that the data which follow are at the very start of the curent track. This can be used to sybchronise the controller.

 

Pin 10: Drive Select 0

 
This signal corresponds to the one on Pin 6, except that i concerns drive 0 (drive A).

 

Pin 12: Drive Select 1

 
As above, except for drive 2 (Drive A).

 

Pin 14: Drive Select 2

 
As above, except for drive 2. Not connected on the St because only two drives are possible.

 

Pin 16: Motor on

 
A high level on this connection starts the motor of all drives and a low stops them.

 

Pin 18: Direction

 
This signal indicates the direction of the next step of the read/write head. If this pin is zero, the direction is in, toward track 79, while a 1 means out, toward track 0.

 

Pin 20: Step

 
A low pulse causes the step motor in the drive to move the read/write head one step in direction specified.

 

Pin 22: Write Data

 
This line carries the serial data which are to be written to the disk.

 

Pin 24: Write Gate

 
This signal selects the data direction. If it is low, the disk is written, while a high signal indecates read. I fthe disk is write protected, no write access is allowed by the drive.

 

Pin 26: Track 0

 
If the read/write head is over track 0, this pin is low.

 

Pin 28: Write Protect

 
A low on this line means that the diskette is write-protected.

 

Pin 30: Read Data

 
The data read from the disk are sent to the computer via this line.

 

Pin 32: Side Select

 
This line selects the desired side of the disk. A low level selects side 1 and a high selects sid 0. This line is unused for single sided drives.

 

Pin 34: Ready

 
A low level on this line indicates that a disk is inserted in the drive and that it is rotating normally. The computer can use this line to determine if the disk has been changed. This line is not connected to the Atari ST.

 

All of these signals are TTL-compatible, meaning that 0-0.4 volts indicates a LO(zero) and a 2.5-5.25 volts means HI(one).