## Input/Output Device Driver Reference

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### Automatic Receive Flow Control (XON/XOFF)

When the physical device driver is enabled for this mode of operation, it transmits an XOFF when its receive queue gets close to full, and an XON when its receive queue is about half full.

The normal mode of Automatic Receive Flow Control causes the ASYNC device driver to stop transmitting all data after it sends an XOFF character until its receive queue lowers to about half full, when it sends the XON character. This behavior is suitable for most devices, but reduces transmit throughput significantly in Full-Duplex (Transmit and Receive) communications scenarios. By setting the Full-Duplex mode of Automatic Receive Flow Control, the physical ASYNC device driver continues to send application data after it sends the XOFF character. See IOCtl ASYNC_ SENTDCBINFO which can be found in the OS/2 Physical Device Driver Reference .

Initial Value Automatic Receive Flow Control is disabled.

First Level Open There is no effect on whether the physical device driver is enabled or disabled for this mode of operation. The state of the physical device driver is reset to show that the last flow control character automatically transmitted was an XON, if it is enabled for this mode of operation.

Close Considerations If the last character automatically transmitted by the physical device driver was an XOFF and a CLOSE request packet is received, the physical device driver automatically transmits an XON, if possible. After processing this close request, the port is not open any more from another open without a close.

Mode Utility Always disables Automatic Receive Flow Control.