Hints For High-Speed Operation; Flow Control; Hardware Cts/Rts Flow Control; Software Xon/Xoff Flow Control - ZyXEL Communications 2864 Series User Manual

Hints for High-speed Operation

The communication software tends to lose incoming serial data when it is doing
block disk data transfers, because the hard disk interrupt priority level is usually
higher than that of the serial port interrupt in the computer. The communication
software can do a hardware flow control during disk data transfer to prevent this.
Enabling a disk cacheing software such as SMARTDRV will help. Please refer to
chapter 21 for more information on high-speed PC-modem communication.
Choice of an efficient assembly coded communication software is also very impor-
tant. When using communication software, use a simple protocol without error
control (the modem has accomplished this job) such as YModem-G and ZModem.
Software with complicated protocol handling will slow down its speed in handling
serial data.

Flow Control

This feature refers to stopping and restarting the flow of data into and out of the
modem's transmission and receiving data buffers. Flow control is necessary so that a
device (computer or modem) does not receive more data than it can handle. The
2864 series modems provide two kinds of flow control methods.

Hardware CTS/RTS Flow Control

This is a bidirectional flow control where CTS and RTS are RS232 signals which
must be available on your computer. When the modem's transmission buffer is
almost full, the modem will drop CTS to signal the DTE that it cannot accept any
more data. Turn ON the CTS to notify the DTE that it can keep sending data to
the modem. On the computer software side, when the receiving buffer of the soft-
ware is almost full, it will drop RTS to signal the modem to stop sending data to the
DTE. Turn ON the RTS and the modem will start sending data again to the DTE.
In asynchronous full-duplex applications, the 2864 always responds to the RTS sig-
nal as a flow control signal. The 2864 defaults automatically to this hardware flow
control setting and it is a better choice.

Software XON/XOFF Flow Control

This is a bidirectional flow control. XON and XOFF character defaults are decimals
17 and 19. These can be changed by modifying the S-Registers 31 and 32. Both the
modem and the DTE will treat XOFF as a signal to stop transmitting data, and will
treat XON as a signal to restart sending data. Modems will not send these charac-
ters received from the local DTE to the remote modem.
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