pc8477b National Semiconductor Corporation, pc8477b Datasheet - Page 50

• Manufacturer Part Number: pc8477b
• Description: Advanced Floppy Disk Controller
• Manufacturer: National Semiconductor Corporation

7 0 Reference Section
7 3 PC8477B INTERFACE IN A PC-AT
The PC8477B interface to the PC-AT bus is simple and re-
quires only an external address decoder All the microproc-
essor inputs and outputs of the PC8477B can be connected
directly to the peripheral bus due to the 12 mA sink capabili-
ty Figure 7-1
header and the signal connections to the AT bus The de-
sign will support 1 2 Meg 1 44 Meg and 2 88 Meg drives
Support for the 2 88 Meg perpendicular drives is accom-
plished with the additional density encoding signal
(DRATE0) on floppy header pin 6 This interface solution will
support perpendicular drives with the encoding scheme list-
ed in Table 7-2 below
Media
1 Meg
2 Meg
4 Meg
TABLE 7-2 Density Encoding
shows the interface with the floppy drive
Data Rate
250 kb s
500 kb s
1 Mb s
(Continued)
HD
X
0
1
FIGURE 7-1 8477B in PC-AT System
ED
0
0
1
50
The HD signal is floppy header pin 2 and the ED signal is
header pin 6 This standard scheme is supported by a num-
ber of perpendicular drive manufacturers Some new per-
pendicular drives are using an auto media sense for density
selection These drives will not require either the HD or ED
signals Here the data rate is determined optically by the
drive due to the hole in the disk
The only use of the 16L8 PAL is address decoding for the
proper floppy address range The primary range is 3F0 – 3F7
while the secondary address range is 370 – 377 Selection
between can be accomplished with a jumper if needed The
address lines A9 – A3 and AEN are input to the PAL from the
peripheral bus The following equation can be used for the
primary range
In this design we have used 1 kX pull-up resistors on the
floppy drive interface If the intended design is to be used
with external drives or long cabling or if 5 25 disk drives
are to be supported 150 kX pull-ups should be considered
CS
e
( AEN A9 A8 A7 A6 A5 A4
TL F 11332 – 24
A3)