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File name Widget_Servo_Functional_Objective.pdf

WIDGET SERVO FUNCTIONAL OBJECTIVE I. BASIC SERVO FUNCTIONS Widget servo-control functions are handled by a Z8 microprocessor. The Z8 handles all I/O operations, timing operations and communication with a host controller. Control functions to the Z8 Servo Controller are made through the serial I/O. The following commands for the Widget servo are: A. HOME - not detented, heads off data zones located at the inner stop. B. RECAL - detented at one of two positions. 1. FORMAT RECAL: 32, -0, +3 tracks from HOME use only during data formatting. 2. RECAL: 72, -0, +3 tracks from HOME use to initialize home pOSi- tion after power on or following an access or any other error. c. SEEK - coarse track positioning of data head to any desired track location. D. TRACK FOLLOWING - heads are detented on a specific track location and the device is ready for another command. E. OFFSET - controlled microstepping of fine position system during TRACK FOLLOWING (two modes). 1. COMMAND OFFSET - direction and amount of offset is specified to the servo. 2. AUTO OFFSET - command allows the servo to automatically move off track by the amount indicated by the embedded servo signal on the data surface (disk). F. STATUS - command can read servo status. G. DIAGNOSTIC - not implemented. See Table 1 for the actual command description. With the present com- mand structure a SEEK COMMAND can be augmented with an OFFSET COMMAND. Upon completion of a seek, the offset command bit is tested to determine if an offset will occur following a seek (either auto or command offset). When a SERVO ERROR occurs the Z8 SERVO will attempt to do a short RECAL (ERROR RECAL). Two attempts are made by the system to do the ERROR RECAL function. If either of the two RECAL operations t'erminate successfully D the protocol status will be SERVO READY, SIO READY and SERVO ERROR. Should the ERROR RECAL fail then the system will complete the error recovery by a HOME function. The two OFFSET commands will be described. First COMMAND OFFSET is a pre- determined amount of microstepping of the fine position servo. Included in the OFFSET BYTE (STATREG) bit B6=0 is a COMMAND OFFSET. Bit B7=1 is a ~~rd offset step (toward the spindle); B7=O is a reverse step_ In the ~~it B6=1 the OFFSET command is AUTO OFFSET. AUTO OFFSET command normally occurs during a write operation. When the HDA was initially forma ted at the factory special encoded servo data was written on each track "near" the index zone. The reason for this follows: Normal coarse and fine position information for the position servos