Viper 60s Theory Of Operation And Maintenance Manual page 137

VIPER SCSI 60,
t
25 &
t
50 THEORY OF OPERA TION AND MAINTENANCE
6.2.3 Read Circuits
The read chain circuits condition the signal being read from the tape. A dual input preampli-
fier and multiplexer is provided for each read heads 0 and 1.
• Gain of the preamplifier is detem1ined and a variable resistor adjusts the head
inputs so that the output is properly balanced.
• Head selection is determined by a forward/reverse signal and the output of the
dual preamplifier/multiplexer is conditioned by a seven-pole differentiating low-
pass filter. This filter changes amplitude peaks into zero-crossings and also filters
unwanted noise frequencies out of the data frequency range.
• The conditioned read signals are applied to a circuit consisting of a post amplifier
and data pulse detector. Gain of the post amplifier is determined by the gain adjust
resistor. The channel-to-channel equalization is achieved by adjusting a resistor.
IX Balance is adjusted to correct for the differential offset voltage of the post
amplifier.
• The output of the post amplifier is AC coupled and attenuated into the read
comparator (zero-crossing detector) and write comparator (threshold detector)
circuits. The threshold detector is triggered with 25% amplitude or 4% amplitude,
depending upon whether the drive is in read-after-write or read only mode of
operation. By qualifying data at 25% threshold during read-after-write, sufficient
margin is provided to insure that the data will be recovered using a 4% threshold
during read only.
• The thresholds are selected by a voltage divider network. When the controlling
transistor is on, the network provides 4% read only threshold. When it is oft the
network provides a 25% read-after-write threshold.
• Data pulse timing is accomplished by a one-shot multivibrator time-domain filter.
The multivibrator is triggered by the zero-crossing detector on every zero crOSSing.
As the multivibrator times out the threshold detector is sampled to determine if
it
too has been triggered.
If it
has, then a second one-shot multivibrator
is
triggered,
which is the read data pulse. The time constant for the time-domain filter
is
carefully selected to insure that unwanted zero crossings do not become false data
transitions. The time constant for the read data pulse is not critical but it is
maintained so it does not exceed 50% of the data window.
6-6