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Administration and Management
2. See if a modem is still attached:
3. Wait for the modem to go off hook and initiate an XMODEM transfer:
If the repeater receives a valid modem response, it knows that a modem
is connected to the COM serial port. The repeater does not know if the
modem is operating at its highest possible baud rate. For example, if the
modem is plugged in just before the 300 Baud initialization string is
issued, the modem remains at 300 Baud. (It is not recommended to
transfer a 180KB Flash image file at 300 Baud if the modem supports a
higher baud rate). Consequently, unless the modem is already connected
at 9600 Baud, the repeater re-issues the 9600 Baud modem initialization
string and then continues to search at each consecutively lower Baud
rate until it detects the highest speed modem supported.
If the repeater does not receive a valid modem response, the connection
algorithm restarts and the repeater firmware reattempts an XMODEM
transfer (Step 1).
Once a modem connection is established, the repeater checks to see if
the modem is still attached by sending an initialization string every
minute. If the modem does not respond, the connection algorithm
restarts, searching for a null modem cable XMODEM transfer (Step 1).
When the repeater's COM serial port modem answers the incoming call,
its Carrier Detect (CD) line is asserted. After the repeater sees an active
CD, it delays 30 seconds and then repeatedly sends the following sync
bytes until an XMODEM transfer starts or CD goes inactive.
Repeater sends a C and waits 10 seconds for a response.
Repeater sends a NAK and waits 10 seconds for a response.
If CD goes inactive, the repeater checks to see if the modem is still
attached (Step 2).