DIGITAL OUTPUT REGISTER (DOR)
Address 3F2 READ/WRITE
The DOR controls the drive select and motor enables of the disk interface outputs. It also contains the
enable for the DMA logic and a software reset bit. The contents of the DOR are unaffected by a software
reset. The DOR can be written to at any time.
7
6
5
4
3
2
1
0
MOT
EN3
MOT
EN2
MOT
EN1
MOT
EN0
DMAEN nRESE DRIVE DRIVE
T
SEL1
SEL0
RESET
COND.
0
0
0
0
0
0
0
0
impedance state. This bit is a logic "0" after a
reset and in these modes.
BIT 0 and 1 DRIVE SELECT
These two bits are binary encoded for the drive
selects, thereby allowing only one drive to be
selected at one time.
PS/2 Mode: In this mode the DRQ, nDACK, TC
and FINTR pins are always enabled. During a
reset, the DRQ, nDACK, TC, and FINTR pins will
remain enabled, but this bit will be cleared to a
logic "0".
BIT 2 nRESET
A logic "0" written to this bit resets the Floppy disk
controller. This reset will remain active until a logic
"1" is written to this bit. This software reset does
not affect the DSR and CCR registers, nor does it
affect the other bits of the DOR register. The
minimum reset duration required is 100ns,
therefore toggling this bit by consecutive writes to
this register is a valid method of issuing a software
reset.
BIT 4 MOTOR ENABLE 0
This bit controls the MTR0 disk interface output. A
logic "1" in this bit will cause the output pin to go
active.
BIT 5 MOTOR ENABLE 1
This bit controls the MTR1 disk interface output. A
logic "1" in this bit will cause the output pin to go
active.
BIT 3 DMAEN
PC/AT and Model 30 Mode:
Writing this bit to logic "1" will enable the DRQ,
nDACK, TC and FINTR outputs. This bit being a
logic "0" will disable the nDACK and TC inputs,
and hold the DRQ and FINTR outputs in a high
BIT 6 MOTOR ENABLE 2
The MTR2 disk interface output is not. (Always 0)
BIT 7 MOTOR ENABLE 3
The MTR3 disk interface output is not. (Always 0)
Table 6 - Drive Activation Values
DRIVE
DOR VALUE
1CH
0
1
2DH
21
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