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N O V A T E L W I R E L E S S T E C H N O L O G I E S L T D .
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A T C o m m a n d S p e c i f i c a t i o n - E x p e d i t e a n d M e r l i n
W i r e l e s s I P M o d e m s
DTR Control Description
The Data Terminal Ready (DTR) line is used by the modem as a qualifier for its operation in a data mode,
TCP, UDP, SLIP or PPP. The DTR line is to be asserted before entering a data mode to indicate to the
modem that the host computer is available for operation. The DTR line is de-asserted to terminate a data
session. The modem will not sustain a data mode session without DTR being active. The use of DTR
can be overridden by setting register S211 to 1: the equivalent of asserting DTR permanently.
The DSR Signal is driven by the modem to indicate the existence of a data session. When DTR is
asserted by the local host, the modem will respond with the DSR signal to indicate that the modem has
entered data mode. DSR is de-asserted to indicate that the modem has exited data mode.
For Listen mode applications, the modem must see DTR asserted when the TCP Session request is
received. If DTR is not asserted the session request will be rejected. When the DTR signal is asserted,
the modem will issue a RING followed by a CONNECT to indicate the beginning of a incoming data
session. Here RING indicates the source of the session as being a remote server while the CONNECT
indicates the transition from AT Command mode to Data mode. . In this mode, the NRM will “Listen” for
TCPP packets with port numbers which match that specified with the NRM’s IP address (re: S110). Any
TCP packets received in AT-CDPD mode, or packets received in TCP-Data mode without a matching port
number, are rejected by the NRM with no indication to the Host. DTR can be used to terminate the
session or the remote server can initiate the termination.
A session may be terminated by the Host using the DTR line. The remote host may also terminate a
session. Connection status is maintained on the DSR line. Once the session has been terminated, the
NRM returns to AT-CDPD mode. Note that session termination is the only method to return to AT-CDPD
mode
.
The DTR line de-asserted can also be optionally used to power the modem off in addition to the
termination of the session. If the S211 register is set to 4, then the de-asserting of the DTR line will make
the modem terminate any current data session, perform a de-registration with the network and power itself
off.
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The Hayes compatible escape sequence is not supported.