Parallel Direct Cable Connection
Parallel Direct Connection
The Parallel Direct Connection is the second solution
to transfer data from one computer to another. The cable required
is slightly more complicated as it has more wires that need to be
connected, but the speeds you will get from it will make it well worth
the time and effort required to make the cable.
Most people would know the parallel Direct Cables as
"Laplink" cables. You get one when you buy the Laplink program
or PCAnywhere, it's usually a yellow cable, but you'll be able to
make your own by the time you finish reading this page.
Because of the variety of parallel (LPT) ports, 4 to
be precise, but we use the same cable for everyone one of them. We
will have a look at them all to make sure we cover everything :)
Now, as far as speed's concerned, with a standard LPT
port you're looking at around 40 to 60 KB per second whereas with
the faster LPT ports you should expect something around 1MB per second
! Whichever way you see it, it's a huge improvement in comparison
to the serial cable (Null modem cable).
Let's have a quick look at the way data is transferred
over a parallel link, this will help us understand why it's also a
lot faster than the serial method of transfer:
This diagram shows a parallel transfer. In serial transfer
there is one block of data moved at a time, whereas with parallel
and, more specificaly in our example, there are 4 data blocks moved
at a time. Parallel ports transmit data simultaneously over multiple
lines and are therefore faster than serial.
If you're having difficulties understanding the diagram
just think of a 4 lane highway, which is our parallel cable, where
4 cars at a time are moving whereas the serial cable is like a one
lane highway with one car at a time moving. Hope that helps :)
What does the parallel port
(LPT) look like ?
The picture below shows a parallel port, also known
as LPT port, of a new computer.
With new computers, you will always find the LPT port
right above the two COM ports and it's usually colour coded purple.
No matter what type of LPT port you have, they all look the same,
it's the electronic characteristics which changes amongst the 4 different
types of LPT ports and that's transparent to everyone. All LPT ports
are female DB-25 connectors.
So what are the different LPT
ports ?
Before we get stuck into the pinouts of the LPT port,
let's have a look at the different types of LPT ports available. Again,
depending on the LPT port, you would expect different speed rates:
Because it might seem a bit confusing at the begining,
I have include a bit more techincal information on the various ports
to help you understand more about them. To keep it simple, I have
categorised and colour coded them to show which ports match the table
above:
4 bit ports
The port can do 8 bit byte
output and 4 bit nibble input. These
ports are often called "unidirectional"
and are most commonly found on desktop bus cards (also called IO expansion
cards, serial/parallel cards, or even 2S+P cards) and older laptops.
This is still the most common type of port, especially on desktop
systems. 4 bit ports are capable of effective
transfer rates of about 40-60 KBytes per second
in typical devices but can be pushed upwards of 140
KBytes/sec with certain design tricks.
8 bit ports
These ports can do both 8 bit
input and output and are sometimes called "bidirectional
ports" but that term is often misused by vendors to refer
to 4 bit ports as well. Most newer laptops
have 8 bit capability although it may
need to be enabled with the laptop's vendor-specific CMOS setup function.
This is discussed below. A relatively smaller percentage of LPT bus
cards have 8bit capability that sometimes must be enabled with a hardware
jumper on the board itself. True 8 bit ports
are preferable to 4 bit ports because
they are considerably faster when used with external devices that
take advantage of the 8 bit capability.
8 bit ports are capable of speeds ranging
from 80-300 KBytes per second, again
depending on the speed of the attached device, the quality of the
driver software and the port's electrical characteristics.
EPP ports
Can do both 8bit input and output at ISA bus speeds.
These ports are as fast as 8 bit bus
cards and can achieve transfer rates upwards of 600
KByte per second. These ports are usually used by non-printer
peripheral devices such as external CDROMs, tape drives, hard drives,
network adaptors and more.
ECP ports
Can do both 8 bit input
and output at bus speeds. The specification for this port type was
jointly developed by Microsoft and Hewlett-Packard. ECP
ports are distinguished by having DMA capability, on-board FIFOs at
least 16 bytes deep, some hardware data compression capability and
are generally featured more than other ports. These ports are as fast
as 8 bit bus cards and can achieve transfer
rates upwards of 1 Mbyte per second and
faster on PCs whose buses will support it. The design is capable of
faster transfer rates in the future.
Laplink cable is used to link two PCs with MSDOS
6.0 or later very effectively by using INTERSVR.EXE
(on Host) and INTERLNK.EXE (on GUEST)
PCs. But it can also be used to data-transfer at faster speed with
DCC Feature of Win9x/Me/2000.
Let's now have a quick look at the pinouts of an LPT
port:
The Cable
As explained, there are different LPT ports, but the
cable used is the same for all types of LPT ports. Depending on your
computer bios LPT settings you will be able to achieve different speed
transfers as outlined in the table above.
The picture below clearly shows the pinouts of the required
cable:
One wire should be attached to the metal body of the
Male pins on both sides, this is also shown as the "metal
body" on the diagram.
Now, because I understand how much trouble someone can
fall into when trying to create a cable and get it to work properly,
I have included the DirectParallel
Connection Monitor Utility, for all the DCC users to troubleshoot
and test DCC connection and cable on both computers. It provides detailed
information about the connection, the cable being used for the connection,
the I/O mode (4-bit, 8-bit, ECP, EPP), the parallel port types, I/O
address, and IRQ.
And that pretty much finishes the discussion on Parallel
Cable Connections !
