Centronics connectors
(9)What are Centronics connectors?
Parallel communication
In electronic engineering, this term refers to parallel ports, i.e. an interface type used in older generation PCs to connect peripherals. Unlike serial communication, where one bit is transmitted at a time, parallel ports transmit multiple bits simultaneously (in parallel), so the name comes from the data transfer method. As a result, parallel ports require multiple data lines on cables and port connectors, and are typically larger than modern serial ports which only require a single data line.
There are many different parallel port types, but this term is often used to refer to a printer port (Centronics port). For many years, this had been the industry standard, until it was standardised in the late 1990s as IEEE 1284, which defined bi-directional Extended Parallel Port (EPP) and Extended Function Port (ECP) specifications.
At present, the standard has been extended with the Universal Serial Bus (USB) and Ethernet devices. The parallel port interface is not used on new computers, due to the widespread use of USB devices and printers with Ethernet or Wi-Fi connectivity.
IEEE 1284
IEEE 1284 is a standard that defines bi-directional communication interfaces used mainly in PCs to connect such peripherals as printers, scanners, plotters, etc. This standard was developed in 1994 by Centronics Unidirectional Port to ensure backward compatibility with the Centronics Unidirectional Port used since the 1970s.
The bus contains 8 data lines, 4 control lines and 5 status lines. Power supply lines are not included. Bus lines are bi-directional (Centronics is uni-directional), signal levels on the lines correspond to TTL levels; the IEEE 1284 interface facilitates data transmission over distances of up to 5 m if the signal and earth lines are twisted, otherwise up to 2 m. The data transfer is acknowledged and has a maximum speed of approximately 80% – 3 MB/s. IEEE1284 does not come with the hot-plug functionality, so disconnecting a cable from the port when the power supply is activated may, in some cases, result in damaging circuits managing parallel data transfer.
Centronics connectors and cables
IEEE 1284 specifies several cable quality standards and three connector types comprising a socket and plug:
- Type A: 25-pin DB-25 connector for host connection
- Type B: Mikrotom (i.e. 36-pin Centronics) for peripheral connection
- Type C: 36-pin Mini Centronics (smaller version of the Type B connector)
Two cable types are also specified:
- IEEE1284-I: used for IEEE1284-A and IEEE1284-B connectors.
- IEEE1284-II: for the IEEE1284-C connector.
The IEEE 1284 daisy-chain specification facilitates connecting up to eight devices to one parallel port. The requirement for a daisy-chain connection is that the device must come with two parallel ports, i.e. one input port and one output port.
Centronics connectors – scope of application
Centronics connectors have numerous specific applications. Some of them are compatible only with peripherals supporting the IEEE 1284 standard. Other are designed to operate with devices using the Small Computer Systems Interface (SCSI). Peripherals without Centronics connectors are frequently equipped with Universal Serial Bus (USB) or Bayonet Neil-Concelman (BNC) connectors.
Although this type of connector is a standard that is practically no longer used in modern computers, it is still applied in many electronic designs that employ a parallel port instead of USB and DB-9 ports. Programming it is easier and faster.
Parallel ports were originally designed to connect a printer to a computer. However, they can be used for numerous other applications, for example to control a mobile robot. There are many such industrial and hobbyist designs available on the Internet. Parallel ports are often used by amateur electronic engineers mainly due to the simple structure of hardware (they operate in line with the n-bit state machine principle) and the ease of development of control software (the port comes with a set of operating and control registers, and software operation is extremely simple). The only disadvantage of a Centronics port is the fact that it needs more transmission lines.
Centronics socket types
Centronics 36 connector
This 36-pin Centronics connector comes with two rows of pins, containing 18 pins each. 36-pin connectors are often used in parallel applications, particularly in printers, etc. They are also known as IEEE-1284 Type B connectors.
Centronics 50 connector
This 50-pin Centronics connector comes with two rows of pins, containing 25 pins each. It is equipped with a locking latch. 50-pin Centronics connectors are most frequently used in legacy SCSI-1 devices (scanners, mass storage devices, etc.). A miniature Centronics 50 connector version is also available for voice and data transmission purposes. It is commonly known as a Telco connector.
Micro Centronics 36
This 36-pin Centronics connector comes with two rows of pins, containing 18 pins each. It is locked in place by a compressive clip. It is similar to the regular 36-pin Centronics connector, however, it is half its size. It can be described as a Mini Delta Ribbon (MDR), also known as an IEEE Type-C connector. This interface is often used in parallel multifunction devices (printers/scanners/copiers/fax machines).
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