A parallel port is a type of interface found on computers (personal and otherwise) for connecting various peripherals. In computing, a parallel port is a parallel communication physical interface. It is also known as a printer port or Centronics port. The IEEE 1284 standard defines the bi-directional version of the port, which allows the transmission and reception of data bits at the same time.
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The Centronics Model 101 printer was introduced in 1970 and included the first parallel interface for printers.[1] The interface was developed by Robert Howard and Prentice Robinson at Centronics. The Centronics parallel interface quickly became a de facto industry standard; manufacturers of the time tended to use various connectors on the system side, so a variety of cables were required. For example, early VAX systems used a DC-37 connector, NCR used the 36-pin micro ribbon connector, Texas Instruments used a 25-pin card edge connector and Data General used a 50-pin micro ribbon connector.
Dataproducts introduced a very different implementation of the parallel interface for their printers. It used a DC-37 connector on the host side and a 50 pin connector on the printer side—either a DD-50 (sometimes incorrectly referred to as a "DB50") or the block shaped M-50 connector; the M-50 was also referred to as Winchester.[2][3] Dataproducts parallel was available in a short-line for connections up to 50 feet (15 m) and a long-line version for connections from 50 feet (15 m) to 500 feet (150 m). The Dataproducts interface was found on many mainframe systems up through the 1990s, and many printer manufacturers offered the Dataproducts interface as an option.
IBM released the IBM Personal Computer in 1981 and included a variant of the Centronics interface— only IBM logo printers (rebranded from Epson) could be used with the IBM PC.[4] IBM standardized the parallel cable with a DB25F connector on the PC side and the Centronics connector on the printer side. Vendors soon released printers compatible with both standard Centronics and the IBM implementation.
IBM implemented an early form of bidirectional interface in 1987. HP introduced their version of bidirectional, known as Bitronics, on the LaserJet 4 in 1992. The Bitronics and Centronics interfaces were superseded by the IEEE 1284 standard in 1994.
Before the advent of USB, the parallel interface was adapted to access a number of peripheral devices other than printers. Probably one of the earliest devices to use parallel were dongles used as a hardware key form of software copy protection. Zip drives and scanners were early implementations followed by external modems, sound cards, webcams, gamepads, joysticks, external hard disk drives and CD-ROM drives. Adapters were available to run SCSI devices via parallel. Other devices such as EPROM programmers and hardware controllers could be connected via the parallel port.
For consumers, the USB interface—and often Ethernet—has effectively replaced the parallel printer port. Many manufacturers of personal computers and laptops consider parallel to be a legacy port and no longer include the parallel interface. The guidelines for Microsoft's Windows Logo Program "strongly discourages" systems builders from including parallel ports.[5] USB-to-parallel adapters are available that can make parallel-only printers work with USB-only systems.
Traditionally IBM PC systems have allocated their first three parallel ports according to the configuration in the table below.
| PORT NAME | Interrupt # | Starting I/O | Ending I/O |
|---|---|---|---|
LPT1 |
IRQ 7 |
0x378 |
0x37f |
LPT2 |
IRQ 5 |
0x278 |
0x27f |
LPT3 |
IRQ 2 |
0x3bc |
0x3bf |
If there is an unused LPTx slot, the port addresses of the others are moved up. (For example, if a port at 0x3bc does not exist, the port at 0x378 will then become LPT1.)[6] The IRQ lines, however, remain fixed (therefore, 0x378 at LPT1 would use IRQ 7). The port addresses assigned to each LPTx slot can be determined by reading the BIOS Data Area (BDA) at 0000:0408.
Bit to Pin Mapping for the Standard Parallel Port (SPP):
| Address | MSB | LSB | |||||||
|---|---|---|---|---|---|---|---|---|---|
| Bit: | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 | |
Base (Data port) |
Pin: | 9 | 8 | 7 | 6 | 5 | 4 | 3 | 2 |
Base+1 (Status port) |
Pin: | ~11 | 10 | 12 | 13 | 15 | |||
Base+2 (Control port) |
Pin: | ~17 | 16 | ~14 | ~1 |
~ indicates a hardware inversion of the bit.
In versions of Windows that did not use the Windows NT kernel (as well as DOS and some other operating systems), programs could access the parallel port with simple outportb() and inportb() subroutine commands. In operating systems such as Windows NT and Unix (NetBSD, FreeBSD, Solaris, 386BSD, etc.), the microprocessor is operated in a different security ring, and access to the parallel port is inhibited, unless using the required driver. This improves security and arbitration of device contention. On Linux, inb() and outb() can be used when a process is run as root and an ioperm() command is used to allow access to its base address; alternatively, ppdev allows shared access and can be used from userspace if the appropriate permissions are set. The cross-platform library for parallel port access, libieee1284, also is available on many Linux distributions and provides an abstract interface to the parallel ports of the system. Access is handled in an open-claim-release-close sequence, which allows for concurrent access in userspace.
Pinouts for parallel port connectors are:
| Pin No (DB25) | Pin No (36 pin) | Signal name | Direction | Register - bit | Inverted |
|---|---|---|---|---|---|
| 1 | 1 | Strobe | In/Out | Control-0 | Yes |
| 2 | 2 | Data0 | Out | Data-0 | No |
| 3 | 3 | Data1 | Out | Data-1 | No |
| 4 | 4 | Data2 | Out | Data-2 | No |
| 5 | 5 | Data3 | Out | Data-3 | No |
| 6 | 6 | Data4 | Out | Data-4 | No |
| 7 | 7 | Data5 | Out | Data-5 | No |
| 8 | 8 | Data6 | Out | Data-6 | No |
| 9 | 9 | Data7 | Out | Data-7 | No |
| 10 | 10 | Ack | In | Status-6 | No |
| 11 | 11 | Busy | In | Status-7 | Yes |
| 12 | 12 | Paper-Out | In | Status-5 | No |
| 13 | 13 | Select | In | Status-4 | No |
| 14 | 14 | Linefeed | In/Out | Control-1 | Yes |
| 15 | 32 | Error | In | Status-3 | No |
| 16 | 31 | Reset | In/Out | Control-2 | No |
| 17 | 36 | Select-Printer | In/Out | Control-3 | Yes |
| 18-25 | 19-30,33,17,16 | Ground | - | - | - |
Inverted lines are true on logic low. If they aren't inverted, then logic high is true.
Pin 25 on the DB25 connector might not be connected to Ground on modern computers.
In early parallel ports the data lines were unidirectional (data out only) so it was not easily possible to feed data in to the computer. However, a workaround was possible by using 4 of the 5 status lines. A circuit could be constructed to split each 8-bit byte into two 4-bit nibbles which were fed in sequentially through the status lines. Each pair of nibbles was then re-combined into an 8-bit byte. This same method (with the splitting and recombining done in software) was also used to transfer data between PCs using a laplink cable.
Hardware IC chips:
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