Specifications

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Parallel Communication
In the Beginning
The original 8-bit parallel port was developed by IBM in 1981 as
a faster interface to dot matrix printers than the then standard
one-bit serial port. The parallel port greatly increases transfer
speeds by using an 8-wire connector that transmits the eight
bits in a byte of data simultaneously, thus sending an entire byte
of data in the time it takes to send a single bit in a serial system.
This byte of data is supplemented by several other handshaking
signals, each sent on its own wire, which ensure that data transfer
takes place smoothly.
The major drawback to the original parallel port or Standard
Parallel Port (SPP) was that it allowed for communication in only
one direction--computer to printer. While there were wires which
the printer could use to indicate its status to the computer, it
could do no more than put a positive or negative charge on
these wires. (See chart below for a comparison of parallel
protocols.) This arrangement effectively limited the parallel port's
potential and precluded it from being used in bidirectional
communication such as is required for external storage devices.
The EPP Advantage
IBM soon realized the advantages to be gained from enabling
the parallel port for bidirectional communication. While early
bidirectional efforts did indeed provide for two way transfer, they
did little to make the parallel port a viable alternative for high
speed data transfer.
To address this problem, a new standard for parallel
communications, known as IEEE 1284 (for the committee which
established it), was approved in 1994. This new standard sought
to correct the major drawbacks to the original parallel port
structure. The first major drawback was that not all parallel
peripherals used the same mechanical interface, and thus the
maximum cable distance between computer and peripheral could
only extend 6 feet. IEEE 1284 sets standards for the cable,
connector and electrical interface that guarantees interoperability
between all parallel peripherals. The specified configuration
ensures that data integrity is maintained, even at the highest
data rates, and at a distance of up to 30 feet.
IEEE 1284 also set design standards for true bidirectional
communication between devices. However, the real advance came
with the Enhanced Parallel Port (EPP). EPP utilizes data cycles
that not only enable bidirectional communication, but also provide
for real-time data transfers by permitting intermixing of block
transfers, read operations and write operations.
The EPP specification also solves the speed problem. Parallel
data transfer was largely performed by software in SPP systems,
and thus data transfer rates were limited to 150 kbps. The new
EPP standard specifies a hardware driven handshake system of
data transfer that allows significantly higher transfer speeds--up
to 2 Mbps. In EPP mode, data transfer takes place as a single
software instruction, and the rest of the transfer is handled by
hardware. This allows an EPP port to function as a 16- or 32-bit
data transfer interface using 8-bit I/O hardware, in effect enabling
EPP peripherals to achieve the same speed and efficiency as
many of their board-level counterparts.
Byte
Byte
b
2
b
7
b
8
b
3
b
5
b
4
b
6
b
1
Byte
b
2
b
7
b
8
b
3
b
5
b
4
b
6
b
1
b
7
b
8
b
3
b
4
Serial Communication
b
1
b
2
b
5
b
6
(Serial vs. Parallel Communication)
b
1
b
2
Byte
b
2
b
7
b
8
b
3
b
5
b
4
b
6
b
1
Byte
b
2
b
7
b
8
b
3
b
5
b
4
b
6
b
1
b
3
b
4
b
5
b
6
b
7
b
8
Parallel Communication
Pin SPP Func
t
1
STROBE: Used by computer to tell pr
character has been transmitted and is
2-9 Data transmission from computer to p
r
10
ACK: Used by printer to tell computer
the transmitted data and is ready for
m
11
BUSY: Used by printeer to regulate d
a
12
PE: Used by printer to tell computer it
13
SELECT: Used to indicate to the com
p
14
AUTOFEED: Printer carriage return
15
ERROR: Printer indicates an unspecifi
16
INIT: Computer initializes printer
17
SELECTIN: Allows a printer to be bro
u
18-25 Ground