Parallel Interface transfer exchange
The parallel port was originally created for
communicating with the printer and thus is called a "printer port".
A
PC may have at most 3 parallel ports, which are named LPT1, LPT2, and LPT3. A
parallel port (printer port) female connector has 25 pins and looks like this:
-------------------------------------------
\ o o o o o o o o o o o o o /
\
o o o o o o o o o o o o /
---------------------------------
At least 8 wires are needed for parallel transfer of 8
bits, but the standard IBM-type printer port uses 17 wires for data transfer
plus some more to ground the system. These extra wires are used in intense
hand-shaking between the PC and the printer.
The computer puts 8 data bits on the 8 data lines and
sends (on a separate line) a so-called strobe signal to the printer to inform
it that 8 bits are ready to be transmitted. The printer reads the strobe signal
and 8 bits and sends an acknowledgement signal on another wire back to the computer.
In this way, the PC printer port does not send data to the printer faster than
the printer can accept it. This data is not in any way synchronized by the
clock signal and goes as fast as it can.
There are five status wires that allow
the printer to let the computer know when it is busy processing the data, or is
out of paper or experiences a paper jam, etc. Four control wires allow the
computer to command the printer to reset itself, to skip the page, etc. This
'standard' parallel port interface can sustain data rates up to 0.15 MB/s,
which is faster than the serial port can reliably operate. To connect a
parallel port to the printer one has to use a special 25-pin to 36-pin cable.
This 'standard' parallel port is currently described in the IEEE (Institute of
Electrical and Electronic Engineers) 1284 standard as a compatibility mode.
There are four more newly created modes, which enhances parallel port
performance.
One of the new modes is called a nibble-mode reverse
operation, and another is the byte-reverse operation. Both of them represent
modifications towards the bi-directional parallel port. The two other modes -
the newest - are the EPP (Enhanced Parallel Port) and ECP (Extended Capability
Port), which allow much faster data transfer rates and are widely used in
parallel port storage devices such as Zip drives, hard drives, CD-ROM drives,
etc.
In EPP design, five more CPU addresses were added to
the initial 3 addresses to allow the CPU to transfer data in just one command
rather than being involved in various steps of the PC- Printer handshaking.
This, along with multiple bytes transfer, available for some EPP hardware
designs, improved data transfer rates to almost 2 MB/s - about 10 times faster
than in the standard 'compatibility' mode. EPP design also allowed block
transfer of data and intermixing of data directions (from and to the computer)
with no additional delays, which made it suitable for such peripherals as the
Zip drive and others.
The ECP interface was meant for even higher data transfer
rates than EPP. It utilizes data compression using the RLE (Run Length
Encoding) protocol, which is most useful for compressing long sequences of
repeated numbers. For example, if the sequence includes 105 zeros, it will be
compressed by transmitting the following statement: “here come 105 zeros"
instead of sending them all one by one.
A relatively loose handshaking protocol, along with
the DMA (direct memory access), made possible even higher data transfer rates
in one direction. To reverse the direction of data transfer, ECP needs several
time-consuming steps. This makes the ECP interface less suitable for the
external storage devices which often intermix the directions of the data transfer
(reading and writing).
Using one port for more than one device used to be
achieved via use of mechanical and electrical switch boxes.
Nowadays, the stackable port devices are used which
have two parallel port connections: one "in" and one "out".
You may now plug a CD-ROM drive to the parallel port of your PC and attach the
printer to the "printer" connector on the CD-ROM.
The device should be intelligent enough to figure out
whether the command from the PC is addressed to the CD-ROM or the printer.
Since this architecture is somewhat manufacturer dependent, it is hard to say
in advance how many devices it is possible to connect in parallel. Sometime,
reversing the order of connections will cause both devices to stop working. If
you are lucky, several devices can be connected like this to the same parallel
port of the PC.
You will be able to find more about this mater here(Uma
das fontes deste material): IEEE (Institute of Electrical and Electronic
Engineers) .
"IEEE, pronounced "Eye-triple-E," stands for the Institute of Electrical and Electronics Engineers. The association is chartered under this name and it is the full legal name. However, as the world's largest technical professional association, IEEE's membership has long been composed of engineers, scientists, and allied professionals. These include computer scientists, software developers, information technology professionals, physicists, medical doctors, and many others in addition to IEEE's electrical and electronics engineering core. For this reason the organization no longer goes by the full name, except on legal business documents, and is referred to simply as IEEE.
The IEEE is dedicated to advancing technological innovation and excellence. It has about 425,000 members in about 160 countries, slightly less than half of whom reside in the United States." Wikipedia
"O Instituto de Engenheiros Eletricistas e Eletrônicos ou IEEE (pronuncia-se I-3-E, ou, conforme a pronúncia inglesa, eye-triple-e) é uma organização profissional sem fins lucrativos, fundada nos Estados Unidos. É a maior (em número de sócios) organização profissional do mundo. O IEEE foi formado em 1963 pela fusão do Instituto de Engenheiros de Rádio (IRE) com o Instituto Americano de Engenheiros Eletricistas (AIEE). O IEEE tem filiais em muitas partes do mundo, sendo seus sócios engenheiros eletricistas, engenheiros da computação, cientistas da computação, profissionais de telecomunicações etc. Sua meta é promover conhecimento no campo da engenharia elétrica, eletrônica e computação. Um de seus papéis mais importantes é o estabelecimento de padrões para formatos de computadores e dispositivos." Wikipedia
Escrito
por Jose Pinto em junho de 2007
