Zenith H-100 Service Manual page 52

5-136
CONTROLLER/DISK-DRIVE LOGIC
DATA SHAPING
Data pulses to the drive are reshaped by U16. a one-shot
mUltivibrator. to 400 ns. Raw data from the drive are
reshaped to 250 ns.
DATA SEPARATION AND PRECOMPENSATION
Data se paration and precompensation are per formed primaril y
by U1. U3. U4. U5. and U22. The data separation circuits
are used when the controller is receiving data from the
disk drive. while the precompensation circuits are used
when the controller is writing data to the disk.
DATA SEPARATION
REA-DDATA (RDD) from the drive couples through U9 and U16
to U1-11 and U22-27 (RAWREAD). RDD contains both data bits
and clock bits. U1 extracts the clock bits and sends them
to U22-26 as RCLK. These pulses are synchronized with RDD.
The 1797 uses the RCLK signal to extract the data bits from
the RAWREAD stream. U22 then formats the data and sends
it to the CPU.
U1 uses a phase-locked loop to keep RCLK in phase with the
incoming data stream. The phase-locked loop consists of
U5. U4. Un. and U1. U5 is a 4-MHz voltage-controlled
oscillator that drives U4 and U13. U4 and U13 select either
4 MHz or 2 MHz. depending on the disk size. If a 5-1/4"
disk is being read. U4-9 is low. This couples the 2-MHz
signal to U1-16. Four megahertz is coupled to U1 for
8"
drives.