Parallel Computer Bus Description and Pinout


Posted on Feb 6, 2014

The IEEE-1284 bus defines a parallel printer bus specification with speeds above 1MBps. IEEE-1284 defines a Point-To-Point asynchronous bi-directional interface. Devices may be either 1284 compatible {the older parallel port devices} or 1284 compliant. The maximum recommended length for a printer cable is 25 feet [up to 32 feet maximum]. Centronic


Parallel Computer Bus Description and Pinout
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s parallel cables run out to 12 feet. The IEEE1284 cable replaced the `Centronics` cable which is obsolete. An IEEE1284 or Centronics interface will be indicated by the printer having an LPT port [line printer terminal], which normally refers to a parallel port. The LPT port would be a Centronics interface before the mid 1990`s or an IEEE1284 after the mid 1990`s. However as of 2005 a printer may not have an LPT port, but instead have a USB port and or a Network port with Ethernet. An Ethernet port will normally be found over an RJ-45 jack. To maintain the compatibility with the original Centronics port which uses open collector drivers, pull-up resistors (Rp) are required to transmit logic high levels. The established common standard resistor values of 2. 2kOhms or 4. 7kOhms. Control and Status lines only require a pull-up resistor (Rp). The data lines and Strobe pin may require a series resistor (Rs) for termination to match the impedance for the 8 bi-directional data lines and the strobe. The series termination value is adjusted so that the addition of the resistor and the driver impedance equal 45 Ohms to 55 Ohms cable impedance. So a driver IC with a nominal impedance of 15 ohms would require a 33 ohm [ standard value ] resistor to comply with IEEE1284. The 1284 standard defines two levels of interface compatibility, Level I and Level II. The Level I interface is defined for products that are not going to operate at...




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