HP LaserJet 5000 HP LaserJet 5000, 5000 N, 5000 GN, and 5000 DN Printers - Us - Page 143

Serial Cable Specifications Serial communication is simply the transmission of data one bit at a time. With just one bit to transmit at a time, data can be transferred with a simple electrical circuit consisting of only two wires. In this fashion, an eight-bit byte is transmitted one bit at a time and the individual bits are reassembled into the original byte on the receiving end. Bit transmission occurs from the least significant bit to the most significant bit. Data Terminal Transmit on pin 2 and receive on pin 3. A printer is typically a DTE device. Equipment (DTE) Data Communications Equipment (DCE) Transmit on pin 3 and receive on pin 2. A modem is typically a DCE device. Asynchronous Communication Asynchronous communication or start/stop transmission is the concept of enclosing a character with a start and stop bit. The RS-232 specification defines the standards for asynchronous serial communication. Parity Parity is a method of error checking at the bit level. HP LaserJet Serial Data Format Transmission is asynchronous, with one start bit, eight data bits and one stop bit. Parity is not used. HP LaserJet printers are DTE devices. Serial Interface Protocol (handshaking) Handshaking is the method by which the flow of data between two devices is controlled. The two methods of flow control that are used by HP LaserJet printers are software flow control, in which one device controls another by the content of the data, and hardware flow control, in which one device can control another by changing the voltage on a wire. Software Flow Control (software handshaking) Xon/Xoff is a data stream handshake protocol that sends Xon (DC1; 11 Hex) to the computer from the printer's transmit data pin when the printer is able to accept data and sends Xoff (DC3; 13 Hex) when the printer is not ready for data. Hardware Flow Control (hardware handshaking) By definition, hardware handshaking is performed when two programs manipulate RS-232 control pins-DTR, DSR, RTS, and CTS to achieve a hardware-based form of flow control. In DTR/DSR handshaking, the sender asserts DTR (Data Terminal Ready) before sending the first character in a stream of data and waits for DSR (Data Set Ready) to be asserted in return. RTS/CTS handshaking is similar, but uses the Request To Send and Clear To Send pins rather than Data Terminal Ready. In either case, the sender delays transmitting data until the receiver is ready. EN Specifications A-19