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Chapter 5 Input/Output Interfaces
Compaq Deskpro 4000N and 4000S Personal Computers
First Edition – September 1997
5-22
5.5.2 ENHANCED PARALLEL PORT MODE
In Enhanced Parallel Port (EPP) mode, increased data transfers are possible (up to 2 MB/s) due
to a hardware protocol that provides automatic address and strobe generation. EPP revisions 1.7
and 1.9 are both supported. For the parallel interface to be initialized for EPP mode, a
negotiation phase is entered to detect whether or not the connected peripheral is compatible with
EPP mode. If compatible, then EPP mode can be used. In EPP mode, system timing is closely
coupled to EPP timing. A watchdog timer is used to prevent system lockup.
Five additional registers are available in EPP mode to handle 16- and 32-bit CPU accesses with
the parallel interface. Address decoding includes address lines A0, A1, and A2.
5.5.3 EXTENDED CAPABILITIES PORT MODE
The Extended Capabilities Port (ECP) mode, like EPP, also uses a hardware protocol-based
design that supports transfers up to 2 MB/s. Automatic generation of addresses and strobes as
well as Run Length Encoding (RLE) decompression is supported by ECP mode. The ECP mode
includes a bi-directional FIFO buffer that can be accessed by the CPU using DMA or
programmed I/O. For the parallel interface to be initialized for ECP mode, a negotiation phase is
entered to detect whether or not the connected peripheral is compatible with ECP mode. If
compatible, then ECP mode can be used.
Ten control registers are available in ECP mode to handle transfer operations. In accessing the
control registers, the base address is determined by address lines A2-A9, with lines A0, A1, and
A10 defining the offset address of the control register. Registers used for FIFO operations are
accessed at their base address + 400h (i.e., if configured for LPT1, then 378h + 400h = 778h).
The ECP mode includes several sub-modes as determined by the Extended Control register. Two
submodes of ECP allow the parallel port to be controlled by software. In these modes, the FIFO
is cleared and not used, and DMA and RLE are inhibited.