PC87351 datasheet(35/86 Pages) NSC

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2.0 Device Architecture and Configuration (Continued)

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2.5

PARALLEL PORT MULTIPLEXER (PPM)

The Parallel Port Multiplexer (PPM) logic allows connection of an external Floppy Disk Drive (FDD) through the Parallel Port

connector (25-pin DIN), instead of, or in addition to, the internal FDD on the normal FDC header. This is done by turning the

Parallel Port pins (normally used by the Parallel Port) into an additional set of FDC pins, while isolating them from the Parallel

Port functionality.

A printer (or any other parallel device) may be exchanged with an external FDD, without turning the system off. The PPM

logic automatically detects whether a parallel device or the FDD is connected, and routes the Parallel Port pins to either the

Parallel Port or the FDC functional blocks, accordingly. See Figure 2-4.

PPM mode is enabled by selecting the PNF pin. When PPM mode is enabled, it is controlled by the PNF input pin, as follows:

• When PNF = 1, PPM is inactive and the Parallel Port pins are assigned Parallel Port functionality.

• When PNF = 0, PPM is active and the Parallel Port pins are assigned FDC functionality.

When PPM mode is disabled, the Parallel Port pins are assigned Parallel Port functionality, regardless of the value of PNF.

The internal FDD (on the normal FDC pins) and the external FDD (on the Parallel Port pins) can be assigned as Drive A and

Drive B respectively, or vice versa.

2.5.1

PPM Mode

The Parallel Port pins function as FDD interface for either drive 1 or drive 3. See Figure 2-4 for the internal routing between

the PPM and FDC, and the Parallel Port and FDC pin-sets when PPM mode is active. The FDC output signals are driven

simultaneously both on the normal FDC pins and on the corresponding Parallel Port pins. The FDC inputs are received from

the FDC pins when either drive 0 or drive 2 are selected, and from the corresponding Parallel Port pins when either drive 1

or drive 3 is selected.

The Parallel Port output signals are isolated from the Parallel Port pins. The Parallel Port input signals, as reflected by STR

Parallel Port.

FDC

Parallel

FDC Inputs

Figure 2-4. PPM Routing

FDC Pins

Parallel Port Pins

Parallel Port Inputs

FDC Outputs

Outputs

PPM Active

PPM Active and

Drive 1 or 3 Selected

Parallel Port Input Values

FDC Outputs

FDC Inputs

Default “Non-Connect”

Port

Parallel Port

PC87351 Datasheet(PDF) 35 Page - National Semiconductor (TI)

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