Intermec Transaction Manager 9560, Руководство пользователя

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9560 Transaction Manager User’s Manual
5-4
Physical Layer
The physical layer describes the hardware components of data transmission. It
sets standards for transmitting voltages and electrical pulses through cables,
connectors, and components. Data transmission at the physical layer is defined
in these three areas:
• Full-duplex or half-duplex transmission
• Parallel or serial transmission
• Asynchronous or synchronous transmission
Half-duplex
Transmits and receives data over a single channel, but in one
direction at a time.
Full-duplex Transmits and receives data simultaneously over two separate
channels within the cable.
Parallel transmission
Sends several data bits across the interface together.
A group of bits moves over several lines at the same time.
Serial transmission
Sends data one bit at a time as a series of pulses. Pulses are
received in a mark (binary 1), or space (binary 0) condition. The speed of the
serial transmission is the baud rate or BPS (bits per second).
You use parallel transmission when the bar code equipment and computers are
located within a few feet of each other. Parallel transmission is much faster
than serial. However, as data lines get longer, noise and interference on the
data lines increase.
Asynchronous transmission
Requires a start bit to mark the beginning of a data
block and a stop bit to mark the end of the data block.
Synchronous transmission
Uses a clock to determine the exact sending or
receiving time for each bit. More than one data character can be sent per frame,
and start and stop characters are not included.
Several different interface cables and connectors meet the standards for the
physical layer. Each has its own strengths and limitations, depending on the
distances between connected devices and the configuration of the equipment.
The 9560 supports RS-232, RS-422, and RS-485 multi-drop interfaces. For more
information, see Chapter 2, “Preparing to Install the 9560.”
Data Link Layer
The data link layer deals with the recognition of data received and the
formatting of data transmitted. Protocols guarantee the control and integrity of
messages between both ends of a communication link. Communication at this
layer is either asynchronous or synchronous.