EN
Specifications A-19
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
Equipment (DTE)
Transmit on pin 2 and receive on pin 3. A printer is typically a DTE device.
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.
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