PC97338VLJ datasheet(144/221 Pages) NSC

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4.6

THE PARALLEL PORT

MULTIPLEXER (PPM)

A PPM is used for a PC, which may have an internal

Floppy Disk Drive (FDD) connected via regular FDC

pins, to interface with either a printer or an external

FDD, via a 25-pin DIN connector.

The printer and external FDD may be exchanged,

without turning the PC off, and without updating the

DOS device tables. The software may assign A to the

FDD connected to the regular FDC pins and B to the

FDD connected to the PPM pins (the default assign-

ment), or vice versa.

The FDC output signals are always connected to the

regular FDC output pins.

The FDC output signals are connected to the PPM

output pins when the PPM is enabled (bits 7 and 6 of

SIRQ3 are 1 and 0, respectively) and a floppy drive is

connected to it (PNF = 0). See Table 72.

The FDC input signals are connected to the regular

FDC pins when either bits 7 and 6 of SIRQ3 are not

equal to 1 and 0, respectively, or when the PNF signal

is active (high).

The FDC input pins are internally multiplexed be-

tween the regular FDC pins and the PPM pins when

bits 7 and 6 of SIRQ3 are 1 and 0, respectively, and

PNF = 0 as follows:

• The PPM pins are connected to the FDC input

signals when DR1=0.

• The regular pins are connected to the FDC input

signals when DR1=1.

To support true floating pins, the pins are back-drive

protected.

When bit 3 of FCR is 1, the PPM pins are floated.

When the PPM is not enabled, the parallel port signals

are connected to the PPM pins. (The PPM is config-

ured when bits 7,6 of SIRQ3 are 10, and bit 1 of SCF3

is 0.)

When bits 7,6 of SIRQ3 are 10, and PNF=1, the par-

allel port signals are connected to the PPM pins.

When bits 7,6 of SIRQ3 are 10, and PNF=0, the FDC

output signals are connected to the PPM pins.

Reading back the DTR or CTR returns their written

values.

Input signals assume their default values (STR regis-

ter): BUSY = 0, PE = 0, SLCT = 0, ACK = 1 and thus

the parallel port module sees cable not connected.

4.7

PARALLEL PORT PIN/SIGNAL LIST

Table 72 on the following page shows the standard

25-pin, D-type connector definition for various parallel

port operations

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