Even at the dawn of the appearance of the first computers, the creators had the task of being able to connect various devices to them. This became especially relevant when computers ceased to occupy entire rooms, and began to fit on a table, that is, they became personal. After all, a computer is not only a means for performing calculations, but also a device whose user can perform many different functions: print text or photos, control various devices, play movies and music, connect with other users from all over the world using a computer network. All this becomes possible when external devices, which are called the common word peripherals, are connected to the computer using special unified connectors called ports.
PC Ports
The ports of a personal computer (otherwise they are also called interfaces) are special devices located on the motherboard of the computer, or additional boards connected to it, which are designed to transfer data between the computer and external devices (printer, mouse, monitor, webcam and etc.). All ports can be conditionally divided into 2 large groups:
- Internal - for connecting devices inside a PC (hard drives, video cards, expansion cards).
- External - for connecting external peripherals (scanner, monitor, keyboard, camera, flash drive).
In this article we will look at one of the types of external port, namely LPT-port, its operating principle, connected devices and modern application.
The advent of the LPT port
Initially, the LPT-port (also called the parallel port) was developed only for connecting printers to a PC, this is reflected even in its name - Line Printer Terminal, a line-by-line printer terminal. But in the future, this interface began to be used to connect other devices: scanners, drives, and even computers to each other.
LPT-port was developed by Centronics, which was engaged in the production of dot matrix printers in the 70s of the last century. But after 10 years, IBM began to use it to connect its high-speed devices. It came to the point that there were several options for this interface from different manufacturers of peripherals.
In the initial version, this port was unidirectional, that is, it could transmit data in only one direction: from a computer to a peripheral device. But this restriction soon ceased to satisfy users, since devices with the ability to transfer data in both directions began to enter the market in large quantities. To do this, various manufacturers offered their improvements - bidirectional, ECP, EPP and others. Until 1994, the international standard IEEE 1284 was adopted.
LPT port scheme
An LPT port is called parallel because data is transmitted using it over several conductors simultaneously, that is, in parallel. This interface has an 8-bit data bus, a 5-bit signal bus and a 4-bit status bus.
Below is the pin diagram of the LPT port.
The principle of operation of the LPT port
In the simplest configuration, to implement the principle of operation of the parallel interface, only eleven wires would be enough, namely: 1 wire per housing (ground), 2 confirmation wires and 8 data transfer wires. But, according to the generally accepted IEEE 1284 standard, each of the eight data transmission wires (2-9) has a separate ground.
During data transfer, both devices must communicate information about their status to each other. This is realized using contacts 18 and 35, which are supplied with a voltage of 0 V or 5 V.
A special STROBE signal is transmitted through conductor 1, indicating that the computer has set a data byte on the line and the printer can begin printing.
On pin 11, a BUSY signal is transmitted to the computer, indicating that the device performs an action (busy), processing the information that is in the buffer.
Contacts 12-14 transmit signals informing control signals about the status of the printer and conflicts in its equipment.
Through conductor 12 to the PC, information is transmitted that there is no paper in the printer. The computer responds to this by transmitting signals on the SELECT and ERROR lines and stops printing.
Information on the status of the printer is transmitted through the conductor 13 to the computer — turned on and ready, or turned off and not ready.
At pin 14, the printer receives a signal about automatic line feed.
Contact 31 (16) transmits a signal to transfer the printer to its initial state and clears the data buffer, i.e., all data is erased from the printer's memory.
Contact 32 (15) transmits all error signals during data transfer. Signals transmitted on this line affect all other contacts and may stop printing. For example, a frequently occurring printer error is Time Out, which occurs when the printer is busy with the same type of work with data and cannot transmit to the PC via the BUSY signal that it is not ready to receive new data. After some time, a Time Out error is transmitted to the computer via the ERROR line and new data is not transmitted. Otherwise, in the absence of an ERROR signal, further data transmission would occur, which would lead to the entire system freezing.
Contact 36 (17) transmits information about the printer’s readiness for work, for example, after resolving the error.
LPT port operating modes
There are several modes of operation of the LPT port, which allows the use of the IEEE 1284 standard:
- SPP (Standard Parallel Port) - is a unidirectional port that combines perfectly with the Centronics interface.
- NibbleMode - the use of this port is the ability to organize bidirectional data exchange in SPP mode by using control lines (4 bits) to transfer data from the peripheral device to the controller.
- Byte Mode is a mode for two-way data exchange, which is used quite rarely. It was used in some older controllers before the adoption of the IEEE 1284 standard.
- EPP (Enhanced Parallel Port) - several well-known companies worked on the development of this port: Intel, Xircom and Zenith Data Systems. In its work, it is a bidirectional port that transmits data at a speed of up to 2 MB / s.
- ECP (Extended Capabilities Port) - this version of the port appeared as a result of the work of two companies: HP and Microsoft. He already had additional features, for example, the possibility of hardware data compression, the presence of a buffer and the ability to work in DMA mode. It also supports bidirectional data exchange (symmetric), the speed of which can be up to 2.5 MB / s.
Configure LPT Port
Configuring an LPT port takes place in two stages: pre-setting the port hardware and the current switching of port modes by application software.
The method and ability to configure the LPT port depends on its location and type of execution. The ports located on the expansion cards are usually configured through jumpers on the boards themselves, and the ports located directly on the motherboard of the computer through the BIOS settings.
The choice of modes directly or via the BIOS alone does not lead to an increase in the speed of data exchange between the PC and peripherals, but serves to enable the driver to select the optimal operating mode. But the drivers of modern devices themselves automatically set the most effective parallel port operating modes, so manual configuration in most cases is no longer required.
Types of LPT Port Implementations
Previously, most motherboard manufacturers placed LPT-port controllers on their products or on the back of the board. There was another layout option. In some cases, it was convenient to place the controller on the board itself - a connector for connecting an external LPT-port strip. But since the advent of faster interfaces for transferring data from motherboards with soldered LPT ports, it has become less and less. Now not even every manufacturer in the range of products has such boards. And then expansion cards connected to more modern interfaces come to the rescue:
- PCI - LPT-port. An adapter between an LPT port and a more modern PCI slot.
- PCI2 - LPT-port (PCI-Ex. 2.0). Adapter between LPT port and PCI-Ex.2.0
- USB - LPT-port. An adapter between the LTP port and the current version of the widely used USB connector.
Current use of the LPT port
Due to the ability to parallel data transfer of such a port, in the 70s - 80s, he established himself as one of the fastest computer interfaces. Therefore, it was even used to connect 2 computers to each other. But this same feature also imposes a limitation on the maximum cable length due to interference in neighboring conductors. The length cannot exceed 5 m, otherwise the signal distortions exceed the permissible for correct data recognition.
With the advent of faster interfaces, the relevance of the LPT port has waned. A second wind was given to him by radio amateurs who use it to control assembled circuits (lighting in the house, light music and other devices).