PC8477B datasheet(50/54 Pages) NSC

50 / 54 page

70 Reference Section (Continued)

73 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-

Figure 7-1

shows the interface with the floppy drive

header and the signal connections to the AT bus The de-

sign will support 12 Meg 144 Meg and 288 Meg drives

Support for the 288 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

TABLE 7-2 Density Encoding

Media

Data Rate

1 Meg

250 kbs

2 Meg

500 kbs

4 Meg

1 Mbs

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

CS e (AEN

A3)

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 525

disk drives

are to be supported 150 kX pull-ups should be considered

TLF11332 – 24

FIGURE 7-1 8477B in PC-AT System

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