Examples (Transparent XON/XOFF)

The following describes the actions performed by the software. The modem performs identical actions on the reverse directions of transmission.

Transmitter Example
Each character in the user data stream is individually examined. If it is any of the six characters $10, $11, $13, $90, $91, or $93, it is exclusive-ORed with the value $21, and prefixed by a DLE character ($10). This is a replacement function as shown in the following table:

Char     Replaced by      

$10      $10 $31          

$11      $10 $30          

$13      $10 $32          

$90      $10 $B1          

$91      $10 $B0          

$93      $10 $B2

The characters are transmitted using the effective parity setting on the DTE-DCE interface; the XOR with $21 preserves the correct value of the parity bit.

If the software wants the modem to stop delivering data, it inserts an XOFF ($13 at current parity setting) in the transmitted data stream at any point, which may be between a DLE ($10) and the subsequent transparent character. The XOFF should be issued BEFORE the buffer is completely full, since there may be a lag of several characters before the modem reacts to the XOFF and suspends delivery. To resume data delivery, the software inserts an XON ($11 at current parity setting) at any arbitrary point. Characters inserted for flow control are NOT passed through the transparency algorithm defined above.

Receiver Example
Each character received from the modem is individually examined. A transparency sequence in progress flag (TSIP-FLAG) is maintained as part of a state machine. The initial value of the TSIP-FLAG is OFF. The following tests and actions should be undertaken in the order listed.

If the character received is $13 or $93 (XOFF with either parity), it is interpreted (regardless of the setting of the TSIP-FLAG) as a request from the modem for the DTE to suspend transmitting data to the modem. The receiver portion of the software must communicate this request to the transmitter portion of the software in a timely manner, since the modem has a limited amount of buffer space to allow additional characters to be received after it sends XOFF (about 64 characters maximum). The XOFF character is discarded from the received data stream. The TSIP-FLAG is not changed.

If the character received is $11 or $91 (XON with either parity), it is interpreted (regardless of the setting of the TSIP-FLAG) as a request from the modem for the DTE to resume transmission of data to the modem, if any is available. The XON character is discarded from the received data stream. The TSIP-FLAG is not changed.

If the character is not an XON or and XOFF, the following steps are performed:

Step 1: If the TSIP-FLAG is ON, the received character is exclusive-ORed with $21, then delivered to the higher layer for further processing (e.g., placed in a receive data buffer). The TSIP-FLAG is then turned OFF.

Step 2: If the TSIP-FLAG is OFF and the character received is $10 or $90 (DLE with either parity), the start of a transparency sequence is indicated. The DLE is discarded from the received data stream, and the TSIP-FLAG is turned ON.

Step 3: If the TSIP-FLAG is OFF and the character received is not a $10 or $90, the character is a normal non-transparent user data character which should be delivered to the higher layer for further processing (e.g., placed in a receive data buffer). The TSIP-FLAG remains OFF (unchanged).

Click here to return to the Contents page.