Information Technology Reference
In-Depth Information
17.5.1 EPP registers
Several extra ports are defined, these are the EPP address register and EPP data register. The
EPP address register has an offset of three bytes from the base address and the EPP data reg-
ister is offset by four bytes. Table 17.4 defines the registers.
Table 17.4
EPP register definitions
Port Name
I/O address
Read/
write
Description
Data register
BASE_AD
W
Status register
BASE_AD +1
R
Control register
BASE_AD +2
W
EPP address port
BASE_AD+3
R/W
Generates EPP address read or write cycle
EPP data port
BASE_AD+4
R/W
Generates EPP data read or write cycle
17.6 ECP
The extended capability port (ECP) protocol was proposed by Hewlett Packard and Micro-
soft as an advanced mode for communication with printer and scanner type peripherals. It
provides a high performance bidirectional data transfer between a host and a peripheral.
The standard provides for two cycle types in both forward and reverse directions:
1. Data cycles.
2. Command cycles which can either be a run length count or a channel address.
It has many advantages over the EPP standard, including:
Standard addresses - ECP has standard register addresses - Figure 17.6 shows that the
addresses from 0778h to 077Ah have been defined for the extra functionality of ECP.
Run length encoding (RLE) - RLE allows for compression. It allows high compression
rates when there is a great deal of repetitive information in a file (typically with graphics
files). A repetitive sequence is identified by a count followed by the repeated byte.
FIFOs for both the forward and reverse channels.
DMA as well as programmed I/O for the host register interface.
Channel addressing - This allows multiple logical devices to be located within a single
physical device. This channel address is passed in the command phase and can support up
to 128 devices (addresses 0 to 127). For example, a single unit could have an integrated
printer, fax and modem. ECP channel address allows them all to be accessed over a sin-
gle connection. Within one physical package, having a single parallel port attached, there
is a printer, fax and modem. This has the advantage that the printer can be busy printing
while the modem can be accessed at the same time.
ECP redefines the SPP signals to be consistent with the ECP handshake. Table 17.5 de-
 
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