V.25bis Indications

Hayes AT result codes are mapped to V.25bis indications as follows.

Hayes Equivalent      V.25bis Indication                                                

OK                    VAL (Valid)                                                       

CONNECT xxxx          CNX (Connection)                                                  

RING                  INC (Incoming call)                                               

NO CARRIER or         CFI AB (Call failure; abort)                                      

NO DIALTONE                                                                             

ERROR                 INV (Invalid)                                                     

BUSY                  CFI ET  (Call failure; engaged tone)

No other V.25bis indications are generated. V.25bis operation is not influenced by the Q and V settings ("quiet"mode and "numeric vs. verbose"). V.25bis indications are always spelled out, and are followed by a CR/LF pair (if async).

The INC result code is issued once for an incoming call. It is issued on the first ring (when S1 counts from 0 to 1). If the call is not answered and the ringing stops, S1 resets after 8 seconds (as usual). A new INC result code is then issued on the next incoming call.

Hayes result codes not shown in the table (such as CARRIER and PROTOCOL) have no V.25bis equivalents. Therefore, the modem produces no output at the time when these result codes would normally be issued.

In asynchronous V.25bis mode, result codes are always transmitted with even parity. In synchronous BSC, they are always transmitted with odd parity. In synchronous HDLC, bit 7 is 0 (in the information field).

Note: V.25bis permits mark, space, or odd parity HDLC in responses.

In HDLC, if a command is received with the poll bit set, the next indication transmitted has the final bit set. Otherwise, the final bit is always zero.

Result codes are not transmitted if DTR is low. If DTR goes low while an result code is being sent, the remainder of the result code is not sent. If DTR goes low during processing of a command, no result code is sent.

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