If a general failure error is not returned by the physical device driver, this function causes data transmission to be stopped by preventing the physical device driver from sending additional data to the transmit hardware.

If Automatic Transmit Flow Control is enabled, this request causes the physical device driver to behave as if it had received the XOFF character. Transmission can be resumed when an XON is received by the physical device driver after a ASYNC_STARTTRANSMIT request is received, or when the physical device driver is told to disable Automatic Transmit Flow Control (if it was enabled).

If Automatic Transmit Flow Control is disabled, then the Function 48h request is required for transmission to be resumed. If, after this request is received, the physical device driver is told to enable Automatic Transmit Flow Control, then transmission is still disabled, but can be re-enabled. See Note 2 of ASYNC_SETDCBINFO.

See ASYNC_GETCOMMSTATUS for other reasons why transmission might be disabled.