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MNP and V.42
Flow control
5.375 x 8.375 ver 2.4.0
Flow control
Serial port flow control
Serial port data flow is controlled by the modem in its
communication with a personal computer.
If the serial port speed is higher than the modem port speed,
this control function sends a transmission halt request to the
personal computer before the buffers in the modem are about
to become full. When the buffers have room to receive data
again, the transmission halt request is canceled and data
transmission from the personal computer is resumed.
The internal modem provides three kinds of serial port flow
control: one determined by software and two determined by
hardware.
XON/XOFF flow control (software)
Flow control is performed by sending XON and XOFF
control codes in the data stream. The XOFF code makes a
transmission halt request. The XON code makes a
transmission restart request.
Computer
DTE
Receive
Buffer
Buffer
Modem
DCE
Phone
Circuit
Modem port
flow control
flow control
Serial port
Transmi
t
Phone line