DKWEB- (Pin #16)
This signal causes a selected drive to start writing data (provided
by DKWDB-) onto the disk.
CHNG- (Pin #11)
A selected drive will drive this signal low whenever its internal
"disk change" latch is set.
This latch is set when the drive is first powered on, or whenever there
is no diskette in the drive. To reset the latch, the system must select
the drive, and step the head. Of course, the latch will not reset if
there is no diskette installed.
MTRXD- (Pin #8)
This is the motor control line for all four disk drives. When the
system wants to turn on a disk drive motor, it first deselects the drive
(if selected), pulls MTRXD- low, and selects the drive. To turn the motor
off, the system deselects the drive, pulls MTRXD- high, and selects the
drive. The system will always set MTRXD- at least 1.4 microseconds before
it selects the drive, and will not change MTRXD- for at least 1.4
microseconds after selecting the drive. An external drives must have
logic equivalent to a D flip-flop, whose D input is the MTRXD signal, and
whose clock input is activated by the off-to-on (high-to-low) transition
of its SELxB- signal. As noted above, both the setup and hold times of
MTRXD-with respect to SELxB- will always be at least 1.4 microseconds.
The output of this flip-flop controls the disk drive motor. Thus, the
system can control all four motors using only one signal on the cable
(MIRXD-).
DRESB- (Pin #10)
This signal is a buffered version of the system reset signal. Three
things can make it go active low):
o System power-up (DRESB- will go low for approximately one second);
o System CPU executes a RESET instruction (DRESB- will go low for
approximately 17 microseconds);
o Hard reset from keyboard (lasts as long as keyboard reset is held
down).
External disk drives should respond to DRESB- by shuffling off their motor
flip-flops and write protecting themselves.
A level of 3.75v or below on the 5v+ requires external disks to write-
protect and reset the motor on line.
- 356 Appendix I -
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