What is a Parallel Port?
A parallel port, also known as a Centronics port, Centronics interface, or Centronics connector, is an external interface that was used to connect peripheral devices such as printers. Epson developed it, and it was widely used on PCs from the early 1980s through the early 2000s. The Centronics port, which is a printer port, is the most popular parallel port. This port is a metal-encased 25-pin (type DB-25) computer interface that is generally found on the rear of IBM compatible computers.
Eventually, USB came into the market that superseded parallel ports. USB offers significantly faster data transfer rates and a smaller connection. It is large in size and slows in speed, and is a hallmark of old computer technology. For keeping the cable in place, it has two screw-in connectors, and it is roughly an inch in width. The parallel port cables that are used to connect the printer to the computer often contain a 36-pin "Centronics 36" connector. Data could be transmitted at a maximum speed of 150 kbps using the original parallel port standard. A DB25 parallel port is shown in the image below.
At a time down multiple strands of copper cable, the parallel port originally was capable of transmitting eight bits of data, and it was a unidirectional type. In 1970, CentronicsData Computer Corporation introduced it. The parallel port could transfer a total of 300Kbits/sec and generally was designed to use with printers. The standard printer port (SPP) or normal port was designed in 1981 as the industry standard for the unidirectional printer port. In 1987, the PS/2 parallel port was created, allowing other peripheral devices such computer mice and keyboards to be attached. The PS/2 had the ability to transmit and receive eight bits of data simultaneously as it was a bidirectional parallel port (BPP).
As the use of printers was increased gradually, then it was necessary to offer bidirectional communication and increase the connection speed. The bidirectional function allowed printers to print by sending messages back to the PC, such as "ready," "printing," and "complete," instead of the PC issuing a "print" command and hoping the print job is successful." It also became capable of other purposes due to its faster transmission speeds, such as Iomega Zip drive. The standard length of a parallel cable to transmit data is 6 feet. You can lose the integrity of the data if you increase the cable length too long. Hewlett-Packard recommended its length be a maximum of 10 feet.
Two new parallel port types, the enhanced parallel port (EPP) and the expanded capabilities port, were launched in 1994. (ECP). The EEP (extended parallel port) was far quicker than conventional parallel ports, with transfer speeds ranging from 500 KBps to 2 MBps. This port was utilised by newer printers and scanners. An 8-bit bidirectional port can also be supported by the expanded capabilities port (ECP). Although it is similar to the enhanced parallel port (EEP), it uses direct memory access. It is used for network adapters or disc drives (non-printer peripherals).
To avoid difficulties of incompatibility with newer different parallel port hardware, the Standard Signaling Method for a Bi-directional Parallel Peripheral Interface standard was implemented in 1994 for Personal Computers (IEEE 1284). The five modes compatibility mode, EPP mode, nibble mode, ECP mode, and byte mode, were specified. All of these modes can handle data transport in either the forward or backward directions, as well as bidirectionally. For the connector, interface, and cable, the IEEE 1284 set standards to ensure that data integrity is maintained. One bit of data is exchanged on each of two wires in a parallel port, which helps to increase the data transfer rate.
Originally the intention of the parallel ports was to provide a connection for printers. For printers, the Centronics Model 101 was the first parallel interface port, which was introduced in 1970. It could transmit data eight bits at a time. This port was not able to receive data but could transmit it. Later it was used for input devices, including printers. The bidirectional parallel port (BPP) is a device that can interface with CD-ROM drives, scanners, hard discs, zip drives, and modems enabling fast data transmission over short distances.
The IEEE standardized the parallel port as "IEEE 1284" in 1994. It also standardized the length of cables, logic voltages, and interfaces. The first version of USB was introduced, but it was not faster as compared to the parallel port. It did, however, have the capacity to deliver electrical voltage to power an external device, as well as other features like a smaller connector. Because it was hot-swappable, a computer could be connected or disconnected safely while it was running. By attaching or detaching a peripheral using an IEEE 1284 port, the device or the PC might be damaged.
When the USB 2.0 standard, which enabled data transfer rates of 480 Mbps, was launched, the parallel port became outdated, and the IEEE 1284 standard slipped into computer history. Thus, the parallel port has been replaced by a universal serial bus (USB). In fact, the parallel interface is excluded by many manufactures. For older personal computers (PCs) and laptops, a USB-to-parallel converter is available for peripheral devices as well as printers.
Where is found the parallel port on a computer?
The parallel port is a part of the motherboard, which is placed on the backside of the computer, as shown in the above picture. Since the introduction of USB, FireWire, and other speedier options, parallel connections are rarely used. Additionally, a parallel port does not come with many new computers and laptops.
Parallel port modes
Depending on the needs and available resources, the computer has the ability to contain the parallel port run at different modes; such modes include ECP, EPP, Nibble Mode, IEEE-1284 (Auto), bidirectional, SPP (unidirectional), and Centronics mode.
Uses of parallel port
In modern times, the USB port widely replaced the parallel port. However, used with the parallel port, there were many hardware devices; such are as follows:
Scanner: A parallel port scanner is a device that was widely used with the parallel port. It is an alternative option to replace SCSI scanners as they are easy to install.
Printer: A printer is a hardware device that is used to print documents, which is the most widely used device for the parallel port.
External drives: External devices, such as the Iomega Zip drive, parallel ports, are popularly used with these kinds of external devices. These devices are portable and can be used with different-different computers because they can be removed from one computer and connected to another.
History of Parallel Ports
A brief history of the parallel port is given below:
In the early 1980s, when computers began to be commonly used in homes and small offices, they came with a port that was used to connect a printer or plotter to the computer. Today, this "Centronics" port is known as Standard Parallel Port or a unidirectional parallel port. Soon, people knew about its speed. As compared to a serial port, it could transfer data theoretically 5 times faster.
But due to the design of the port, there was a bottleneck. In order to transfer data, the standard parallel port had four wires. Although comparing serial ports, the speed of file transfer of parallel ports was fast; they were more expensive. And, the high speed did not justify the additional cost.
Later, IBM released a new standard called "bi-directional when it introduced the PS/2 line of computers. For transferring data, the new port included 8 ports that had transfer speed increased to 150 Kbytes/sec and more. Then, parallel ports were redesigned by many companies to take benefit of their speed. There were multiple devices that appeared in rapid successions, such as CD-ROM drives, hard disks, CCD cameras, scanners, as well as networks devices.
Three companies, Xircom, Intel, and Zenith, developed a new standard called EPP (Enhanced Parallel Port). It came with a new data transfer speed of 1000 Kbytes/sec.
Finally, another standard ECP (Enhanced Capability Port) was created by Microsoft that increased the transfer speed to 2000 Kbytes/sec. Unluckily, all these standards have only partially different.
Parallel ports on Apple computers
On Apple Macintosh computers, SCSI is used as an interface that is parallel. As compared to the parallel port used with IBM-compatible computers, SCSI is more flexible. The parallel port has never been used by Apple computers.
Difference between Serial Port and Parallel Ports
The below table contains the major difference between serial ports and parallel ports: