Digital Equipment RW504-ZA, Service Manual

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6.4.1.1 SCSI Controller
The SCSI controller provides the SCSI interface connection to the host computer.
This interface consists of both the electrical signals and the firmware which
decodes the various commands and messages on the SCSI bus and instructs the
drive to take appropriate action.
6.4.1.2 Data Buffer
The 256-Kbyte data buffer and associated control electronics provide a buffer to
speed match transfers to and from the host computer and the optical disk. The
data buffer provides a cache for read and write operations, optimizing the speed
of these transfers.
6.4.1.3 Formatter/Sequencer
The formatter/sequencer function formats and decodes data for read and write
transfers. During a write function user data is sent via the SCSI bus as bytes (8
bits each). The formatter converts these 8 bits of parallel data into an encoded
serial bit stream that includes all the format and error correction features
required to meet the ANSI and ISO specifications. During reads, the decoder
converts the serial data stream, which includes format and error correction
features, into parallel data.
6.4.2 Servo PCA
The servo PCA includes a digital signal processor (DSP) that controls the actions
of the many servo loops that interact with the optical head and the loader
assembly.
6.4.2.1 DSP Microprocessor and Support/Servo Control Loops
Most of the electronics on the servo PCA is analog circuitry. This circuitry is
designed into the control loops of the following servos: spindle motor/speed,
laser power control, track following, seek/position maintenance, focus actuation,
fine position actuation, and coarse position actuation. Whenever the drive is
performing a read or write operation, all these servos are activated.
Also on the servo PCA are control electronics for the loader assembly. Included
are drivers for the loader and spindle (speed control) motors, sensor circuitry
for the cartridge loaded and write protect detectors, LED drivers for the front
panel LEDs, and control circuitry for the bias magnet subassembly of the loader
mechanism.
Theory of Operation 6–9