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User’s Guide Product Features
2110212 Rev 1.0 Page 35
RTS must remain asserted, indicating that the RI signal can be passed to the host. If RTS is de-
asserted, the RING result is buffered until RTS is asserted, and the RI signal is not generated. The
host would not be advised of the connection request.
The RI signal is toggled on a one-second on : three-seconds off duty cycle; mimicking the ring
pattern of North American wireline telephones. The number of repetitions of this cycle and the
behaviour of the modem will depend on the packet service mode being used.
UDP/TCP
When data arrives from the network, either an accepted UDP packet, or a TCP connection request,
RI is toggled. The modem will also send the RING result once with each connection request.
If the modem is set to auto-answer (S0=1) then the modem will toggle RI once only. The modem
will open the session and expect the local host to wake up. The CONNECT result will be sent
and DCD asserted (respecting RTS flow control). Flow control should be opened promptly; the
modem will continue to hold the session open and buffer incoming data.
The modem will toggle the RI signal up to three times for each connection attempt in cases where
the modem is not set to auto-answer (S0=0). The cycles stop as soon as the host issues the
A (Answer) command. If the host fails to answer:
• UDP will buffer the packet and hold the session pending; subsequent client requests will be
handled according to the setting of the UDP Server Receive Lock (+WS217). When the local
host does answer, the original session is opened and processed.
• TCP will timeout the opening handshake. If the local host attempts to answer after the caller
has given up waiting, the modem will wait for the TCP PAD Timeout (S7) and then issue NO
CARRIER.
The RTS and DTR flow control signals can be used by the host to manage the transition to data
state and buffering in the modem. See Section 4.4.2 above for details on flow control.
SLIP/PPP
For SLIP/PPP, the RI feature is enabled and disabled with the &I (Ring Indicator) command. This
defaults to enabled (set to 1).
Since the modem has no knowledge of the status of individual sessions on the local host’s IP
stack, the modem can only advise the host whenever a message arrives. This means that the
modem will toggle the RI signal whether the host is awake or not, and whether there is an open
client/server session or not.
When the modem has an open data state connection, any incoming messages from the network
will trigger the RI signal to toggle active for 1 second and inactive for three seconds. Note that
this refers to network messages and not necessarily IP packets. The CDPD network may break up
an IP packet into multiple CDPD messages.
The timing of the message arrival and ring indicator toggle will determine how many RI signals
are detected. When a message arrives, the modem will check if the RI signal is already within the
four-second duty cycle. If not then the cycle is started with RI going active. When the four-
second cycle finishes, the modem checks to see if another message is being received. If there is an
active message, then the cycle restarts; otherwise the RI remains off until the next message arrives.
4-Second Duty Cycle
RI
Network Messages
Figure 5-1: RI Timing Diagram.
(Note that /RI is active low)