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Inside Macintosh: Networking With Open Transport / Part 2 - Open Transport Reference

Chapter 31 - Serial Endpoint Reference / Serial-Specific Commands

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I_SetSerialXOff

This command causes the serial port to send an XOFF character. A value of kOTSerialSendXOffIfXOnTrue causes it to be sent only if the endpoint is in the XON state (that is, if the last input flow control character sent was XON), while a value of kOTSerialSendXOffAlways unconditionally sends the character. The following line of code unconditionally sends an XOFF character:

OTIoctl(theSerialEndpoint, I_SetSerialXOff, kOTSerialSendXOffAlways);

Sending the XOFF character unconditionally is equivalent to using a serial driver control call with a csCode value of 26, and sending the XOFF character when an endpoint is in an XON state is equivalent to using a csCode value
of 25.