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Parallel Interface

The de-facto standard for printer communication used to be the parallel interface – sometimes called a "Centronics" interface after the company that originated it. The Centronic Model 101 seven wire solenoid impact printer introduced in 1970 was arguably the first dot matrix printer. Centronics printer business was sold to Genicom in 1987.

The parallel interface uses a lead that usually has two different plugs:

• The printer plug is 36 way – (typically Amphenol 5730360). Nearly half the connections in this plug are used for grounds. This is the "Centronics" plug
• The PC plug is a 25 way male D plug – a few of the grounds are sacrificed to allow this smaller plug to be used.

Data is passed in 8-bit chunks, hence the term parallel interface. This interface can use very little circuitry because it closely matches the internal circuits of both personal computers and printers. There are 8 data wires and usually as many as 8 control wires, including a few that do odd historical tasks like Auto-Feed and Paper End that are often not implemented. The entire PC interface was originally made from about 6 TTL logic chips, although these days it is usually just one aspect of a motherboard support chip.

The parallel interface as first implemented has several limitations.

• Plugs and cable are big and use crude screw and clip locks. Most printers are not small, but clumsy plugs don’t meet modern expectations.

• Cable length is limited to 5 metres or less. Many sources quote 2 or 3 metres but the more relaxed standard generally works. Unless the printer is right next to a PC the allowed lead is often too short.

• The interface can function at up to 2 megabytes per second – but can be much slower with older designs of PC and printer.

• Bi-directional transmission is a relatively recent capability – older PCs and printers often do not support it. Of course most data goes from PC to printer – but return information can show what paper is currently loaded and the progress of the job.

The parallel interface may be technically obsolescent but printer manufacturers were reluctant to drop it from their designs until very ecently. The workings of a parallel interface are just about inherent to microprocessor system design so it costs very little to include it in a printer. It is such a widespread standard that omitting it puts some customers to inconvenience.

Varieties of Port

Parallel Ports actually come in several varieties. There are two  types of printer connector and three behaviours:

Over the years the Centronics port was widely adopted as a connector for printers, but no particular convention emerged about which connector should be used at the computer end.

In 1981 IBM incorporated a parallel port on their PC, but used a 25 pin D socket on the bracket. Since the PC became a standard, so did this connector. IBM also used some simple circuitry as a loopback connector that rather looked as though it could be made to work as a bidirectional bus. Bidirectional use of the data lines often wasn't reliable, but by edefining four of the handshake lines the parallel port could be made to work network adapters, tape drives and other peripherals. IBM put a fully bidiretional port on the PS/2 in 1987.

EPP

In 1992 Xircom and Zenith, two of the leading peripheral makers, introduced the idea of an enhanced Parallel Port - EPP. this required some changes in pin definition and timing. Since port operation can be called as a BIOS service an option to select normal or EPP operation appeared in CMOS setups.

In 1991 a group of printer manufacturers formed a group called the Network Printing Alliance to develop standards for print sharing. EPP triggered interest in further improvements and work was coordinated through the IEEE.

IEEE 1284

In 1994 the parallel interface "morphed" into IEEE1284 - a parallel bus with a range of about 10 metres if the right cable is used. Again there were changes in timing which need to be reflected in BIOS and might be selectable in the CMOS setup.

As defined by IEEE there are 5 possible standards for port operation.
 

SPP or Standard Parallel Port - behave like an old Centronics port, data signals go one way.

Nibble Mode - Bidirectional data signals are supporte by reinterpreting some signal

Byte Mode - Bidiectional signals use the data lines to transfer 8 bits either way

EPP - Enhanced Parallel Port - fully bidierctional

ECP - Enhanced Capability Port - the full range of IEEE1284 bus behaviour.

A PC CMOS is likely to offer SPP, EPP and ECP.

Connectors

 

A connector - this is the 25 pin male D plug normally used at the computer end of a cable

B connector - this is the 36 way male Amphenol or "Centronics" plug.

C connector - a high density "micro" version of the Centronics plug.

The C connector has stayed comparatively rare - which is possibly a shame as it tends to guarantee that the cable is a proper IEEE 1284 construction with 62 Ohm twisted pair construction and full shielding.

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Obsolete

Although it sounds potentially complicated parallel ports usually just work. Many of the bus elaborations envisaged by IEEE1284 are almost never used. USB was devised at around the same time as IEEE 1284. The USB cable is thinner. the connectors simpler and the cost of the protocol in terms of device detection and identification, serialiser / deserialiser  and so forth became trivial.  Almost all recent printers have USB ports and parallel is only used on large devices where  a mixture of operating systems might be in use.

PC motherboards still generally have parallel ports although the idea that they are obsolete is discussed by producers. It costs very little to implement the port on a PC. Most recent notebooks now omit the parallel port although it is often available on port-replicators and USB to parallel convertors are available.

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© Graham Huskinson 2010

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