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

Transmit on pin 3 and receive on pin 2. A modem is typically a DCE

Communications

device.

Equipment (DCE)

 

 

 

Asynchronous

Asynchronous communication or start/stop transmission is the concept

Communication

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

Transmission is asynchronous, with one start bit, eight data bits and one

Serial Data

stop bit. Parity is not used. HP LaserJet printers are DTE devices.

Format

 

 

 

Serial Interface

Handshaking is the method by which the flow of data between two

Protocol

devices is controlled. The two methods of flow control that are used by

(handshaking)

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

Xon/Xoff is a data stream handshake protocol that sends Xon (DC1; 11

Control (software

Hex) to the computer from the printer’s transmit data pin when the printer

handshaking)

is able to accept data and sends Xoff (DC3; 13 Hex) when the printer is

 

not ready for data.

 

 

Hardware Flow

By definition, hardware handshaking is performed when two programs

Control (hardware

manipulate RS-232 control pins-DTR, DSR, RTS, and CTS to achieve a

handshaking)

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.

A-16 Specifications

EN