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DLE
STX
Communicating with a
Modem
Reply Format, SRD Request
The format of a reply to an SRD request is shown in Figure 4.5:
the CMD byte will have the reply bit set
the link type will be set to SRD Request
the link status will indicate success or failure
the LSAP of the slave device (SLSAP) will be included
if the request was successful, the data will be included. Note that
some devices may not return data. The format of data is completely
dependent on how the destination device was implemented. Refer to the
destination device's user manual for this information.
Figure 4.5
SRD Reply Packet Format
DST
SRC
CMD
LINK TYPE
49
01
The handshaking option must be enabled for the KF3 to properly control a
modem.
The KF3 module continually asserts DTR when it is waiting for a call.
Under this condition, the modem will answer a call and assert DSR as
soon as it detects ringing. The KF3 does not monitor the ring indicator in
the RS-232C interface. Once it detects DSR, the module starts a timer
(approximately 10 seconds) and waits for the DCD signal. When the
module detects DCD, communication can start.
If the KF3 does not detect DCD within the 10-second timeout, the
module turns DTR off. This causes the modem to hang up and break the
connection. When the hang-up is complete, the modem turns off DSR.
This causes the module to reassert the DTR line and wait for another call.
This feature protects access to the phone if someone calling a wrong
number reaches this node.
After detecting DCD, the KF3 continues to monitor the DCD line. If DCD
goes off, the module restarts the 10-second timeout. If DCD is not restored
within the timeout, the module initiates the hang-up sequence. This feature
allows the remote node to redial in the event that the connection is lost
through a fault in the phone network.
Chapter 4
Communicating with the KF3
LSTS
SLSAP
DATA...
DLE
ETX
BCC/CRC
4 9
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