Parallel port

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Parallel ports usually used for connecting printers, computers, and other devices that required high bandwidth. The port is also used for interconnection between interior components of the devices. Parallel ports can be transmitted simultaneously over multiple wires. Theoretically parallel port is eight-time faster than a serial port. The serial port sends one bit while at the same time parallel ports send eight bits.

Parallel ports have an issue of crosstalk, especially with long wires.  Due to transfers a data over multiple wires the clock skew has also occurred which is creating synchronization issues.  Mostly the parallel port support data transfer only into one direction.  The common parallel port is a printer port also known as the Centronics port.

The latest version of parallel ports supports bi-directional data transfer. The standard for the bi-directional parallel port is IEEE 1284. The parallel port is now largely changed with the USB ports. Over a parallel cable, we can transmit data up to the length of 6 to 10 feet. If the cable is long then this length, the integrity of the data can be lost.

The cable Centronics Data Computer Corporation introduced the cable in 19970s. The data transfer rate at the time was 300 Kbits per seconds. The cable was introduced for the printer port and known as standard printer port (SPP). In 1987, the PS/2 port was introduced. The PS/2 was a bi-directional port and mostly was used for mouse, keyboard and games handles. The PS/2 was known as the bidirectional parallel port (BPP).  In 1994 the Enhanced parallel port(EPP) and The Extended capabilities port (ECP) was introduced.

The EPP was faster port than older parallel ports, which is supporting 500 KBps to 2 MBps data transfer speed. The ports are also used for printer connectivity. During this time, the five modes of operation were also specified as ECP mode, EPP mode, byte mode, nibble mode and compatibility mode. All the modes must carry data transfer in the forward direction, backward direction or bidirectional. The parallel ports are usually known as LPT1, LPT2, etc. The figure below illustrates the parallel port (DB-25). The parallel port is in the shape of the letter D.

Parallel port 3

Parallel port Pin Detail

The parallel port connector has an 8-bit data bus with a maximum recommended cable length of 10-feet. Cable long than recommended length can create poor connection and data signals. The pin information from pin 1 to pin 25 is the following:

  • Pin 1 – Strobe
  • Pin 2 – Data bit 0
  • Pin 3 – Data bit 1
  • Pin 4 – Data bit 2
  • Pin 5 – Data bit 3
  • Pin 6 – Data bit 4
  • Pin 7 – Data bit 5
  • Pin 8 – Data bit 6
  • Pin 9 – Data bit 7
  • Pin 10 – Ack
  • Pin 11 – Busy
  • Pin 12 – Paper End
  • Pin 13 – Select
  • Pin 14 – Auto Feed
  • Pin 15 – Error
  • Pin 16 – Initialize Printer
  • Pin 17 – Select Input
  • Pin 18 –Bit 0 Return (Ground)
  • Pin 19 –Bit 1 Return (Ground)
  • Pin 20 –Bit 2 Return (Ground)
  • Pin 21 –Bit 3 Return (Ground)
  • Pin 22 –Bit 4 Return (Ground)
  • Pin 23 –Bit 5 Return (Ground)
  • Pin 24 –Bit 6 Return (Ground)
  • Pin 25 –Bit 7 Return (Ground)

Pin1 – It is used for acknowledgement when the signal is low.

Pin 2 to 9 – It is used for data transfer.

Pin 10 – It is providing acknowledgement when data has finished processing. The pin has two states high and low when the pin signal is on the high state, it’s mean ready for more.

Pin 11 – When the signal state is on high, it means that the printer has accepted the data and is processing it. When this signal becomes low and Pin 10 goes high then the connected device will accept additional data.

Pin 12 – When printer jams a paper, the pin signal goes high and the printer stopped working.
Pin 13 – When the signal is high on pin 13, the printer is indicating that it is on-line and ready to print.

Pin 14 – When pin 14 has low signal, then the PC has indicated that the printer inset a line feed after each line.

Pin 15 – In case of an error the printer sends data to the computer that an error has occurred.
Pin 16 – When the signal goes low on pin 16, then the PC initiate request for internal reset.
Pin 17 – When the signal is low on that pin the PC has selected the printer and should in return prepare for data being sent.
Pin 18-25 – Ground.

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