Zenith Z-100 PC series Service Manual page 252

Page
3.5
Circuit Description
When transceiver U348 Pin 1 is in a high state, the direction of data
transfer is from the CPU DO through D7 bus to the onboard DBO
through DB7 data bus. When the data transfer is complete, floppy
controller U30S issues an interrupt request (IRQ6) at pin 18 of U30S,
causing the DMA operation to stop. The IRQ6 signal is buffered and
driven onto the computer backplane bus by pin 14 of U308.
• Disk Drive Selection - Disk drive selection is made through the de ­
coded outputs of U312. Two separate internal disk drives can be
selected to interface with the floppy controller. The CPU places data
onto the onboard DBO through DB7 bus through data transceiver U348.
The status of data bits DBO and DB1 are latched by pins 2 and S
of flip-flop U321 and provide the select A and select B inputs to U312.
The status of bits DB4 and DBS are latched and then ORed by pin
3 of U320 to provide the data input to pin 1 of U312. Similarly, the
status of data bits DB6 and DB7 are latched and then NORed by
pin 13 of U301 to provide the data input to pin 1S of U312. The decoded
disk drive select signal (DSO*, DS1 *, DS2*, or DS3*) is transmitted
to the disk drive through pin 10, 12, 14, or 6 of P301, respectively.
The outputs at pin 3 of U320 and pin 13 of U301 supply the MOTOR*
signal to pin 16 of P301 .
• Beginning of Track - The INDEX* signal from the selected disk drive
is interfaced to the floppy controller through pin 8 of connector P301,
pulled up by pin S of resistor pack RP301, and inverted by pin 8 of
U302. The resulting IDX signal at pin 17 of U30S .indicates the begin­
ning of a track on the disk.
• Read/Write Head Selection - A high head select (HD) output signal
from pin 27 of U30S is inverted by pin 6 of U307 providing the SIDE1 *
signal to pin 32 of connector P301. When the signal at pin 6 of U307
is high, SIDEO is selected.
• Disk Sector Seek - ReadIWrite Modes - The floppy controller in­
itiates either a seek or read/write mode. In the seek mode, the head
within the floppy disk drive can be moved to a different portion of
the magnetic disk. The read/write mode allows data to be read from
a sector on a disk or to be written to a sector on a disk.