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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_SetSerialXOn

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

OTIoctl(theSerialEndpoint, I_SetSerialXOn, kOTSerialSendXOnAlways);

Sending the XON character unconditionally is equivalent to using a serial driver control call with a csCode value of 24, and sending the XON character when an endpoint is in an XOFF state is equivalent to using a csCode value
of 23.