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6. 9.6k DATA COMMUNICATIONS
6.2
About Data Communications
Modem
A modulator/demodulator is used for data communications over a telephone line.
The FELCOM 82 has a modem built into its communication unit. (The MES can
be thought of as a modem.)
AT commands
One of the command sets used to control a modem. For further information, see
paragraph 6.9 AT Commands.
Operation states
The following operation states are observed when the MES is used for data
communications:
Command state
The MES accepts AT commands when in this state. It recognizes the characters
entered via the DTE as a character string. The MES is not connected to any
remote station.
On-line data state
In this state, the MES is performing data communications with a remote station
connected to it. The characters entered via the DTE are sent to that station.
On-line command state
In this state, the MES is connected to a remote station, but still accepts AT
commands. The characters entered via the DTE are recognized as a character
string and are not sent to the remote station.
Note: The MES never enters the on-line data state unless it is connected to a
remote station.
Flow control
The data received from the DTE is stored in the transmit buffer of the MES. If the
data volume sent from the DTE is larger than that sent from the transmit buffer to
the LES, data is gradually stored in the buffer. Once the buffer becomes full with
the stored data, the MES tells the DTE to stop sending data. Accordingly, data is
no longer sent by the DTE. In turn, the data in the buffer is sent to the LES while
the DTE is stopping data transmission. When the buffer becomes empty, the
MES informs the DTE that it accepts data. Now the DTE starts sending data
again. The above procedure for controlling the flow of data is called "flow control."
6-3
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