VT82C686B
Super-I/O I/O Ports
Floppy Disk Controller Registers
These registers are located at I/O ports which are offsets from
“FDCBase” (index E3h of the Super-I/O configuration
registers). FDCBase is typically set to allow these ports to be
accessed at the standard floppy disk controller address range
of 3F0-3F7h.
Port FDCBase+2 – FDC Command.................................RW
Port FDCBase+4 – FDC Data Rate Select......................WO
7
6
5
Motor 3 (unused in VT82C686B: no MTR3# pin)
Motor 2 (unused in VT82C686B: no MTR2# pin)
Motor 1
7
6
5
Software Reset
0
Normal operation................................... default
1
Execute FDC reset (this bit is self clearing)
0
Motor Off
Power Down
1
Motor On
0
1
Normal operation................................... default
Power down FDC logic
........................................always reads 0
4
3
2
Motor 0
0
Motor Off
Reserved
1
Motor On
4-2 Precompensation Select
Selects the amount of write precompensation to be
used on the WDATA output:
000 Default ................................................... default
001 41.7 ns
DMA and IRQ Channels
0
Disable
1
Enable
FDC Reset
0
Execute FDC Reset
010 93.3 ns
1
FDC Enable
011 125.0 ns
1-0 Drive Select
00 Select Drive 0
100 166.7 ns
101 208.3 ns
01 Select Drive 1
1x -reserved-
110 250.0 ns
111 0.0 ns (disable)
1-0 Data Rate
Port FDCBase+4 – FDC Main Status...............................RO
MFM FM
Drive Type
7
6
5
4
Main Request
00 500K 250K bps 1.2MB 5” or 1.44 MB 3”
01 300K 150K bps 360KB 5”
10 250K 125K bps 720KB 3” ................ default
0
Data register not ready
1
Data register ready
Data Input / Output
11
1M illegalbps
0
CPU => FDC
Note: these bits are not changed by software reset
1
FDC => CPU
Port FDCBase+5 – FDC Data.......................................... RW
Non-DMA Mode
0
FDC in DMA mode
Port FDCBase+7 – FDC Disk Change Status................. RW
1
FDC not in DMA mode
7
Disk Change......................................................... RO
FDC Busy
0
Floppy not changed................................ default
0
FDC inactive
FDC active
........................................ always reads 0
Drive 1 Active
1
Floppy changed since last instruction
1
..................................... always reads 1’s
6-2 Undefined
3-2 Reserved
1-0 Data Rate ........................................................WO
00 500 Kbit/sec (1.2MB 5” or 1.44 MB 3” drive)
01 300 Kbit/sec (360KB 5” drive)
1
0
Drive inactive
1
Drive performing a positioning change
10 250 Kbit/sec (720KB 3” drive)
0
Drive 0 Active
11 1 Mbit/sec
0
Drive inactive
1
Drive performing a positioning change
Revision 1.71 June 9, 2000
-48-
Register Descriptions - Super-I/O I/O Ports
ETC [ ETC ]
