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A parallel cable connection is a method that allows data to be transferred from one computer to another. The parallel cable required is slightly complicated as it has more wires than other methods to be connected. However, the transfer speeds achieved make it well worth the time and effort required to make the cable.
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As we'll see below, there are three different types of line print terminal (LPT) ports -- Standard Parallel Port (SPP), Enhanced Parallel Port (EPP) and Extended Capabilities Port (ECP) -- each supporting different speeds and features, but all use the same direct cable connection.
To better understand why the links in a parallel cable connection are much faster than serial links, we need to analyze the way data is transferred. This is clearly shown and explained in the diagram below:
This diagram shows data transfer via parallel ports, and we can see multiple data blocks being simultaneously transferred from one host to another, increasing significantly the overall throughput. Serial ports are capable of transferring one data block per time and are, therefore, unable to match speeds of parallel ports.
What does the parallel port (LPT) look like? The picture below shows a parallel port, also known as an LPT port, on a computer:
With older computers, you will always find the LPT port right above the two COM ports, and it's usually color coded purple. When looking at an LPT port, it is impossible to identify whether the port is an SPP, EPP or ECP type because all three types visually look the same and all LPT ports use DB-25 female connectors.
Examining the different type of LPT Ports
Before we begin analyzing the pin outs of the LPT port, let's have a look at the different type of LPT ports available. The diagram below shows all three LPT ports along with their supported speed and description:
The port can support 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 I/O 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. Four-bit ports are capable of effective transfer rates of about 40-60 Kbps in typical devices, but can be pushed upwards of 140 Kbps with certain design tricks.
These ports can do both 8-bit input and output and are sometimes called bidirectional ports, but vendors often misuse that term, using it 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 8-bit 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. Eight-bit ports are capable of speeds ranging from 80 to 300 Kbps, 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 8-bit 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 Kbps. These ports are usually used by non-printer peripheral devices such as external CD-ROMs, 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 HP Inc. ECP ports are distinguished by having DMA capability, onboard 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 Mbps and above on PCs whose buses will support it. The design will be capable of faster transfer rates in the future.
Let's now have a quick look at the pinouts of an LPT port:
LPT direct connect cable
As explained, there are different LPT ports, but all use the same parallel cable for direct transfer between two hosts. Depending on your computer bios' LPT settings, you will be able to achieve different speed transfers as outlined previously.
The diagram below clearly shows the pinout configuration of the LPT direct connect cable:
Note: 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.
Users attempting to create a LPT direct cable will find the download from Firewall.cx, Direct Parallel Connection Monitor Utility, useful as it provides useful information about the LPT ports on the computer it's run on, plus information about the connection, I/O mode (4-bit, 8-bit, ECP, EPP), the parallel port types, I/O address and IRQ.
Catalog of all 10 cabling lessons
Cabling series, lesson7: Direct cable connection
Cabling series, lesson 8: Serial direct cable connection
Cabling series, lesson 10: USB direct cable connection