Parallel port
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A parallel port is a type of interface found on computers (personal and otherwise) for connecting various peripherals. It is also known as a printer port or Centronics port . The IEEE 1284 standard defines the bi-directional version of the port.
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[edit] Current popularity
For the most part, the USB interface has replaced the Centronics-style parallel port. Also, some printers use an ethernet connection instead. As a result, many personal computer manufacturers (beginning in the early 2000s) have begun omitting the parallel port to save cost. It is now considered to be a legacy port. For laptops, access to a parallel port is still commonly available via a docking station; and adapters are available for USB, etc.
[edit] Uses
- Some printers
- Zip drive
- Some scanners
- Some sound cards
- Some Webcams
- Some gamepads and joysticks
- Peripheral devices such as EPROM programmers
- SCSI devices via a Parallel to SCSI adapter
- Experimental setups using the 12 TTL drivers.
- External CD-ROM/RW drives
[edit] Implementation on IBM personal computers
[edit] Port addresses
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 5 | 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.) 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 | Pin: | 9 | 8 | 7 | 6 | 5 | 4 | 3 | 2 |
Base+1 | Pin: | ~11 | 10 | 12 | 13 | 15 | |||
Base+2 | Pin: | ~17 | 16 | ~14 | ~1 |
~ indicates a hardware inversion of the bit.
[edit] Program interface
In versions of Microsoft Windows that did not use the Windows NT kernel (as well as MS-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 accesses 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.
[edit] Pinouts
Pinouts for a DB25 connector are:
| Pin No (DB25) | Signal name | Direction | Register - bit | Inverted |
|---|---|---|---|---|
| 1 | nStrobe | Out | Control-0 | Yes |
| 2 | Data0 | In/Out | Data-0 | No |
| 3 | Data1 | In/Out | Data-1 | No |
| 4 | Data2 | In/Out | Data-2 | No |
| 5 | Data3 | In/Out | Data-3 | No |
| 6 | Data4 | In/Out | Data-4 | No |
| 7 | Data5 | In/Out | Data-5 | No |
| 8 | Data6 | In/Out | Data-6 | No |
| 9 | Data7 | In/Out | Data-7 | No |
| 10 | nAck | In | Status-6 | No |
| 11 | Busy | In | Status-7 | Yes |
| 12 | Paper-Out | In | Status-5 | No |
| 13 | Select | In | Status-4 | No |
| 14 | Linefeed | Out | Control-1 | Yes |
| 15 | nError | In | Status-3 | No |
| 16 | nInitialize | Out | Control-2 | No |
| 17 | nSelect-Printer | Out | Control-3 | Yes |
| 18-25 | Ground | - | - | - |
[edit] Monodirectional parallel ports
In early parallel ports the data lines were monodirectional (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.
[edit] See also
[edit] References
- Parallel Port Description and programming
- Axelson, Jan (2000). Parallel Port Complete. Lakeview Research. ISBN 0-9650819-1-5.
- The (Linux) Parallel Port Subsystem by Tim Waugh
af:Parallelle poort
ar:منفذ متوازي
br:Porzh a-stur
de:Parallele Schnittstelle
es:Puerto paralelo
fr:Port parallèle
id:Port paralel
it:Porta parallela
lv:Paralēlais ports
ms:Port selari
nl:Parallelle poort
ja:パラレルポート
pl:Port równoległy
pt:Interface paralela
ru:LPT-порт
sv:Parallellport
tr:Paralel port
zh:并行端口
