Zenith Z-100 Series, Technical Manual

Page: 364 / 493

Page 6.25
DETAILED CIRCUIT DESCRIPTION
The reset state for the 1797 is a 03H in the command register,
a 01H in th e s e c tor register, a 0 i n t h e N o t R e ad y bi t ( b it
7) of the status register, and a restore command execution.
The reset state of the control latch makes all outputs of the
latch equal to 0. For the phase lock loop control, the reset
state makes the phase four (phi 4) input equal to 0.
Next, the U26 Q outputs are set to 1, which sends an RDY
(ready) signal to the CPU and which provides part of the qual-
ification needed for Read and Write enabling through AND
gate D of U27.
Also on power up, the WG (write gate) output from the 1797
t o the 5.25"and 8" d r ives is k ep t h ig h b y Q 2 a n d Q 3 u n t i l
the supply voltage is at o r a b ove 4 v o lts at R 25. W hen the
supply reaches 4 v o l ts , Q 2 a n d Q 3 a r e b i a se d n ea r t h e ir
operating region and w ill conduct when W G i s m ad e a ctive
at the 1797. This circuitry is designed to prevent accidental
writing on diskettes if they are left in the drives when the power
is turned on or off. However, diskettes should still not be left
in the drives when the power is switched on or off because
there is no guarantee that the drives will not accidentally write
onto a diskette, without regard to the state of the write gate
line.
Read and Write Functions
Reading and writing with the Floppy Disk Controller Card in-
volves transferring three types of information: data which can
be read or written, status signals, and control signals. Status
signals can only be read and control signals can only be writ-
ten.