All printers need to communicate with the host computer. Communications is handled in the interface section. Printers connected to mainframe computers generally have a communications protocol that is designed for the computer, such as NTDS fast or NTDS slow interface. Smaller printers used with personal computers will have either serial or parallel interfaces. The   most   widely   accepted   serial   interface   is “Recommended Standard-232” or simply RS-232. The most widely used parallel interface is the  Centronics standard parallel interface. RS-232 Serial Interface RS-232 was developed by the Electronics Industry Association (EIA) to be a universal serial interface standard for any serial device such as  a modem,  printer, or keyboard. For a printer  to properly  receive  serial  data,  the parallel bytes  that the computer works with must be converted into a serial data string. Once the data string is  received  by  the  printer,  it  must  be  reconverted  to parallel data for the printer to use. These conversions are accomplished by a special circuit called a  universal asynchronous receiver/transmitter (UART). The UART can perform both parallel  to serial and serial to parallel conversions. UARTs do not need extra control lines to control the flow of data, so the UART never knows when anew character is arriving. To send a data word, the UART must attach from two to four extra bits. First, the UART inserts a binary ZERO to represent a The text that you are reading now is printed in portrait mode.  In  landscape  mode,  the  page  is  rotated  90 degrees and the data is printed across the length of the page. Using standard paper, portrait mode is aligned 8.5 inches wide × 11 inches long; in landscape mode the paper is aligned 11 inches wide × 8.5 inches long. Orientation  is  selectable  on  some  dot  matrix printers, ink jet printers, and all laser printers. TOPIC 2—BASIC PRINTER CHARACTERISTICS All printers have the same function, that is to print data  on  paper.  The  method  they  use  to  put  the information on paper varies with the type of printer. This section covers areas of the printer that are common to most printers: interface, control, paper feed, and power  supply. INTERFACE SECTION start bit. The next seven or eight bits represent the actual  data  code.  Although  some  printers  still  work with seven data bits, eight bits is the standard found on most printers today. After the data code is sent, the parity of the data is checked and a parity bit maybe added. Whether a printer uses even or odd parity is determined by the manufacturer and is set up when the printer is connected to a computer. To end the data word, the UART adds one or two stop bits. Configuring the UART for a printer is accomplished by setting a number of dip switches of the circuit board. The RS-232 interface cable is connected to the computer  and  printer  by  a  DB-25  sub-miniature connector. The DB-25 connector is a 25-pin D-type connector. Although the connector has 25-pins, serial communications with software  handshaking needs as few as three of the pins connected. Handshaking  signals  are  signals  that  control  the printer. Software handshaking uses the ASCII codes such as  XON/XOFF   and ETX/ACK.  Hardware handshaking uses an additional line to indicate  data terminal ready (DTR) to the computer’s data set ready (DSR)  pin.  When  hardware  handshaking  is  used, the printer cannot send data to the computer. Centronics Parallel Interface The Centronics parallel interface uses a 36-pin Centronics connector at the printer end of the cable and a DB-25 subminiature connector at the computer. The parallel interface is an eight-bit, two-way interface between  the  computer  and  the  printer.  When  the computer sends data to the computer, it places the data on the data lines and sets a strobe signal. The strobe signal indicates to the printer that the data is ready for transfer. When the printer samples the data, it will set the acknowledge line to tell the computer it has sampled the  data. CONTROL  SECTION The control section of a printer directs all printer operations.  This  section  receives  and  decodes computer data from the interface section. If the data contains characters to be printed, the control section determines what character it is and when to activate the print mechanism. The print mechanism can be a print hammer, a series of print wires, a laser beam, or some other mechanism. The control section receives signals from various parts of the printer as to the presence of paper, carriage position, and print head temperature. 12-5