View source for IEEE 1284

{{short description|Standard for parallel peripheral interfaces, known as the Centronics port}}
{{Use dmy dates|date=March 2022}}

{{Multiple issues|
{{more footnotes|date=July 2016}}
{{Lead too short|date=January 2024}}
}}

[[File:IEEE 1284 36pin plughead.jpg|thumb|An IEEE 1284 36-pin male [[micro ribbon]] printer cable connection. The computer side normally uses a [[D-subminiature|DB-25]] port instead of this connector]]

'''IEEE 1284''', also known as the '''Centronics port''', is a standard that defines bi-directional [[parallel communications]] between computers and other devices. It was originally developed in the 1970s by [[Centronics]] before its [[Institute of Electrical and Electronics Engineers|IEEE]] standardization.

== History ==
[[File:Centronics-36F.jpg|thumb|An IEEE 1284 36-pin female on a circuit board]]

In the 1970s, [[Centronics]] developed the now-familiar printer [[parallel port]] that soon became a [[de facto standard|''de facto'' standard]]. Centronics had introduced the first successful low-cost seven-wire print head{{citation needed|date=January 2017}}, which used a series of solenoids to pull the individual metal pins to strike a ribbon and the paper.

A dot matrix print head consists of a series of metal pins arranged in a vertical row. Each pin is attached to some sort of actuator, a solenoid in the case of Centronics, which can pull the pin forward to strike a ribbon and the paper. The entire print head is moved horizontally in order to print a line of text, striking the paper several times to produce a matrix for each character. Character sets on early printers normally used 7 by 5 "pixels" to produce 80-column text.

The complexity of printing a character as a sequence of columns of dots is managed by the printer electronics, which receives character encodings from the computer one at a time, with the bits transferred serially or in parallel.<ref>[http://chiclassiccomp.org/docs/content/computing/Centronics/101_101A_101AL_102A_306_SpecificationsInterfaceInformation.pdf Centronics 101, 101A, 101AL, 102A, and 306 Printers: Specifications and Interface Information]</ref> As printers grew in sophistication, and the cost of memory dropped, printers began adding increasing amounts of buffer memory, initially a line or two, but then whole pages and then documents.

The original port design was send-only, allowing ''data'' to be sent from the host computer to the printer. Separate pins in the port allow status information to be sent back to the computer. This was a serious limitation as printers became "smarter" and a richer set of status codes were desired. This led to an early expansion of the system introduced by [[Hewlett-Packard|HP]], the "Bitronics" implementation released in 1992. This used the status pins of the original port to form a 4-bit parallel port for sending arbitrary data back to the host.

A further modification, "Bi-Directional", used the status pins to indicate the direction of data flow on the 8-bit main data bus; by indicating there was data to send to the host on one of the pins, all eight data pins became available for use. This proved adaptable, and led to the "Enhanced Parallel Port" standard, which worked like Bi-Directional mode but greatly increased the signalling speeds to 2&nbsp;[[Megabyte|MB]]/s, and later the "Extended Capability Port" version increased this to 2.5&nbsp;MB/s.

In 1991 the [[Printer Working Group|Network Printing Alliance]] was formed to develop a new standard. In March 1994, the IEEE 1284 specification was released. 1284 included all of these modes, and allowed operation in any of them.

== Characteristics ==
[[File:IEEE 1284 printer cable, type AB-0135.jpg|thumb|right|An IEEE 1284 compliant printer cable, with both [[D-subminiature|DB-25]] and 36-pin [[Micro ribbon|Centronics]] connectors]]

The IEEE 1284 standard allows for faster throughput and bidirectional data flow with a theoretical maximum throughput of 4&nbsp;MB/s; actual throughput is around 2&nbsp;MB/s depending on hardware. In the printer venue, this allows for faster printing and back-channel status and management. Since the new standard allowed the peripheral to send large amounts of data back to the host, devices that had previously used [[SCSI]] interfaces could be produced at a much lower cost. This included [[Image scanner|scanners]], [[tape drive]]s, [[hard disk]]s, computer networks connected directly via parallel interface, network adapters and other devices. No longer was the consumer required to purchase an expensive SCSI card—they could simply use their built-in parallel interface.

The parallel interface has since been mostly displaced by [[local area network]] interfaces and [[USB|USB 2.0]].

== Modes ==
IEEE 1284 can operate in five modes:

* Compatibility Mode, also known as Centronics standard or Standard Parallel Port (SPP), is a uni-directional implementation with only a few differences from the original Centronics design. This mode is almost exclusively used for printers. The only signals that the printer can send back to the host are some fixed-meaning status lines that signal common error conditions, such as the printer running out of paper.
* Nibble Mode is an interface that allows the device to transmit data four bits (a [[nibble]]) at a time, (re)using four of the status lines of Compatibility Mode for data. This is the Bi-tronics mode introduced by [[Hewlett-Packard|HP]] and is generally used for enhanced printer status. Although never officially supported with these, Nibble Mode works with most of the pre-IEEE-1284 Centronics interfaces as well.
* Byte Mode, also known as "Bi-Directional" (although all modes except Compatibility Mode are in fact bi-directional), is a half-duplex mode that allows the device to transmit eight bits at a time using the same data lines that are used for the other direction. This mode is supported on a minority of pre-IEEE-1284 interfaces as well, such as those built into the [[IBM PS/2]] computers; because of this, it is sometimes unofficially called the PS/2 mode.
* Enhanced Parallel Port (EPP) is a half-duplex bi-directional interface designed to allow devices like printers, scanners, or storage devices to transmit large amounts of data while quickly being able to switch channel direction. EPP can provide up to 2&nbsp;MB/s bandwidth, approximately 15 times the speed achieved with normal parallel-port communication with far less CPU overhead.<ref>{{patent|EP|0640229|Buxton, C.L. / Kohtz, R.A. / Zenith Data Systems Corp.: ''Enhanced parallel port.'' filing date 15 May 1992}}</ref>
* Extended Capability Port (ECP) is a half-duplex bi-directional interface similar to EPP, except that PC implementations use [[direct memory access]] (usually [[Industry Standard Architecture|ISA]] DMA on channel 3) to provide even faster data transfer than EPP by having the ISA DMA hardware and the parallel port interface hardware handle the work of transferring the data instead of letting the CPU do this work. Many devices that interface using this mode support [[Run-length encoding|RLE]] compression. ECP can provide up to 2.5&nbsp;MB/s of bandwidth, which is the natural limit of 8-bit ISA DMA.<ref>{{cite web |url=http://www.nor-tech.com/solutions/dox/ieee1284_parallel_ports.pdf |title=IEEE 1284: Parallel Ports |author=LAVA Computer MFG Inc |date=2002 |archive-url=https://web.archive.org/web/20061115015448/http://www.nor-tech.com/solutions/dox/ieee1284_parallel_ports.pdf |archive-date=15 November 2006 }}</ref> An ECP interface on a PC can improve transfers to pre-IEEE-1284 printers as well, by reducing the CPU load during the transfer; however, the transfer in that case is unidirectional.

Most recent computers that include a parallel port can operate the port in ECP or EPP mode, or both simultaneously.

IEEE-1284 requires that bi-directional device communication is always initiated in Nibble Mode. If the host receives no reply in this mode, it will assume that the device is a legacy printer, and enter Compatibility Mode. Otherwise, the best mode that is supported on both sides of the connection is negotiated between the host and client devices by exchanging standardized Nibble Mode messages.

== Connectors and cables ==
[[File:Mini-Centronics 36 pin with Micro-Centronics 36 pin.jpg|thumb|Mini-Centronics 36-pin male connector (top) with Micro ribbon 36-pin male Centronics connector (bottom)]]
[[File:Micro-Centronics 36 pin with Mini-Centronics 36 pin.jpg|thumb|Mini-Centronics 36-pin male connector (right) with Micro ribbon 36-pin male Centronics connector (left).]]

An IEEE-compliant cable must meet several standards of wiring and quality. Three types of connectors are defined:

* Type A: [[D-subminiature|DB-25]] 25 pin, for the host connection.
* Type B: [[Centronics]] (officially called "[[micro ribbon|Micro Ribbon]]") 36-pin, for the printer or device connection.
* Type C: [[Mini-Centronics]] (MDR36 or HPCN36) 36-pin, a half-pitch, smaller alternative for the device connection that has not proven popular.

There are two kinds of IEEE 1284 cables:

* IEEE 1284-I: uses IEEE 1284-A and IEEE 1284-B connectors.
* IEEE 1284-II: uses IEEE 1284-C connectors.

In IEEE 1284 Daisy Chain Specification, up to eight devices can be connected to a single parallel port.

All modes use [[Transistor–transistor logic|TTL]] voltage [[logic level]]s, which limits the possible cable length to a few meters unless expensive special cables are used.<ref>[http://massmind.org/techref/io/parallel/faq0994.htm IBM Parallel Port FAQ/Tutorial]</ref>

== Standards ==
* IEEE 1284-2000: Standard Signaling Method for a Bi-directional Parallel Peripheral Interface for Personal Computers
* IEEE 1284.1-1997: Transport Independent Printer/System Interface- a protocol for returning printer configuration and status
* IEEE 1284.2: Standard for Test, Measurement and Conformance to IEEE 1284 (not approved)
* IEEE 1284.3-2000: Interface and Protocol Extensions to IEEE 1284-Compliant Peripherals and Host Adapters- a protocol to allow sharing of the parallel port by multiple peripherals (daisy chaining)
* IEEE 1284.4-2000: Data Delivery and Logical Channels for IEEE 1284 Interfaces- allows a device to carry on multiple, concurrent exchanges of data

== Typical color codes ==
The following are the typical colors found on 25-pin IEEE 1284 cable leads.{{Citation needed|date=May 2016}}

{| class="wikitable"
! Pin !! Color !! Alt. color
|-
| 1 || red || 
|-
| 2 || pink/red || 
|-
| 3 || brown ||  
|-
| 4 || orange ||  
|-
| 5 || light-blue/yellow || 
|-
| 6 || light-blue/red || 
|-
| 7 || light-blue || 
|-
| 8 || blue || 
|-
| 9 || light-blue/black || green/blue
|-
| 10 || green || 
|-
| 11 || yellow || 
|-
| 12 || pink/orange || 
|-
| 13 || gray ||
|-
| 14 || gray/green || 
|-
| 15 || pink/blue || orange/white
|-
| 16 || pink/black || brown/white
|-
| 17 || light blue/blue || light blue/green
|-
| 18 || blue-white || 
|-
| 19 || green/black || green/red
|-
| 20 || pink/white || yellow/black
|-
| 21 || gray/black || 
|-
| 22 || white/black || gray/yellow
|-
| 23 || purple || 
|-
| 24 || pink || 
|-
| 25 || white || 
|-
| NC || white/yellow || white/green
|-
| All || white/purple || red/black
|}

== See also ==
* {{annotated link|IEEE 1394}}
* {{annotated link|IFSP}}
* {{annotated link|List of interface bit rates}}
* {{annotated link|Universal Serial Bus}}

== References ==
{{refs}}

== External links ==
* [https://web.archive.org/web/20040803084003/http://www.fapo.com/1284int.htm Warp Nine Engineering's introduction to the IEEE 1284-1994 standard]
* [http://retired.beyondlogic.org/epp/epp.htm Interfacing to the Enhanced Parallel Port]
* [http://www.undocprint.org/formats/communication_protocols/ieee_1284 Undocumented Printing Wiki – IEEE 1284 Standards]
* [http://zone.ni.com/devzone/cda/tut/p/id/3466 IEEE 1284 – Updating the PC Parallel Port]
* [http://ckp.made-it.com/ieee1284.html IEEE 1284 summary]
* [http://pinouts.ru/ParallelPorts/ParallelECP_pinout.shtml IEEE 1284 ports pinouts]
* [http://www.theiling.de/parport/ Signal Diagrams for IEEE 1284 Protocol]
* [http://hw-server.com/parallel-port-lpt-ieee-1284 Parallel port – LPT (IEEE 1284)]
* [http://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/parport/daisy.c Linux C code for 1284.3 Daisy Chaining using Command Packet Protocol (CPP) ]
* [http://www.delorie.com/djgpp/doc/rbinter/id/11/21.html INT 17H – EPP BIOS – INSTALLATION CHECK]
* [http://www.delorie.com/djgpp/doc/rbinter/it/32/6.html Use EPP BIOS entry point to call Vectored EPP Services]

{{Computer-bus}}
{{IEEE standards}}

[[Category:Computer buses]]
[[Category:IEEE standards]]
[[Category:Legacy hardware]]