Intermec Transaction Manager 9560, Руководство пользователя

Страница: 79 / 309

Data Communications and Operating Options
5-5
5
Asynchronous protocols at this level perform error detection. For synchronous
protocols, this layer detects starting and ending flags and generates a cyclical
redundancy check (CRC). This layer also identifies whether the transmission
channel is dedicated (point-to-point) or shared (multipoint), and if transmission
is full- or half-duplex.
Full-Duplex Protocols Versus Half-Duplex Protocols
At the data link layer, protocols are described as either full-duplex or half-
duplex. A full-duplex protocol allows messages to be received and transmitted
simultaneously. Your system must have separate, dedicated channels for
receiving and transmitting data to use a full-duplex protocol. Also, the host
and reader must have the necessary hardware (interrupt structure) to support
receiving and transmitting data simultaneously. You define a full-duplex
protocol by disabling both the solicitation sequence (Poll and Select) and
AFF/NEG handshakes.
Point-to-Point and similar user-defined protocols are the only Intermec
protocols that are full-duplex. These protocols do not support retransmission of
data messages. The reader transmits data when commanded to by the
operator, but the reader receives data from the host at any time.
A half-duplex protocol requires that only one device transmit data at a time
over a shared or dedicated channel. You can use a half-duplex protocol over a
full-duplex medium, but only one device at a time can transmit data. You
define a half-duplex protocol by defining either the POL character or the
AFF character.
Full-Duplex Devices Versus Half-Duplex Devices
Devices are full-duplex or half-duplex depending on their ability to receive and
transmit data simultaneously. Both hardware and software design determine if
a device is full or half-duplex. A full-duplex design requires a hardware UART
(universal asynchronous receiver/transmitter) with double-buffered transmit
and receive buffers. The UART signals the CPU when a character is received or
when the transmit buffer is ready for another character. The software must
perform the transmit or receive functions when the UART signals the CPU
with an interrupt.
The DLE Character and XON/XOFF Flow Control
You use the DLE (data link escape) character to send protocol commands or
parameters that you want to be treated as data. The DLE must precede the
transparent command or parameter. The following protocol parameters are
considered transparent if DLE is enabled: RES, REQ, SOM, TX EOM1,
TX EOM2, and XON/XOFF.
You can send the DLE, EOR (end of record), and EOF (end of file) characters as
data if each is preceded by an DLE.