Silicon Laboratories SI2493/57/34/15/04 - Manual (Page 237)

AN93
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9.2.2. Procedure
To enable the SMS features on the Si24xx, the host sends “AT+FCLASS=256” to the modem prior to an SMS call.
To enable the hardware escape pin functions, the host would set HES with the command “AT:U70|8000”.
After setting the other U-registers according to the configurations of the Originate and Answer modems, the host
can dial an SMS call using the command “ATDTxxxx;” (where xxxx is the number to be dialed) or answer an SMS
call with “ATDT;”. The semi-colon at the end of the command places the modem into command mode after dialing.
The modem responds to the host with “OK”. The host then puts the modem into transmit or receive SMS data
mode.
Many SMS POS protocols perform handshaking using CAS and CAS ACK. CAS is a two-tone signal (2130 Hz /
2750 Hz); CAS ACK is a DTMF ‘D’. The terminal modem connects to the server, which sends CAS tones until it
times out or the terminal modem replies to the server with CAS ACK. The threshold for the modem’s CAS tone
energy detector is set by CTDT, which has a default value of 500h. The CAS tone must have a minimum duration
of 30 ms to guarantee detection.
CAS detection is enabled by setting the detector threshold UD4 to a value other than 7FFFh. When the CAS
detector is enabled, FSK CS/MARK detection will run after CAS is detected. The modem will time out if CAS is not
detected. (When implementing SMS POS protocols that don’t use CAS and CAS ACK, the CAS tone detector must
be disabled with the command AT:UD4,7FFF.)
Once CAS is detected, the modem responds with CAS ACK. The duration of CAS ACK is 60 ms.
CASRPT selects Auto or Manual mode for transmitting CAS ACK.
Auto mode (CASRPT = 0): Auto mode reduces the delay between CAS and CAS ACK. After an AT+FRM=200
command, the modem detects the CAS tones and sends CAS ACK without reporting CAS detection to the host.
Manual mode (CASRPT = 1): Manual mode allows the host processor to have more control. A modem reports to its
host with the string "CAS" when it detects CAS tones. The modem waits for the host to reply with the ASCII
character 'D', then it responds to the other modem with CAS ACK. If the host sends any character other than 'D',
the modem returns to the command mode immediately without sending CAS ACK and replies to the host with
<DLE><ETX>. This allows the host to send any DTMF digit.
The host can then send an AT+FTM command or an
AT+FRM command. If the host doesn't send any characters or commands, the modem will time out (as set by UD2)
and go back on-hook without sending CAS ACK.
9.2.2.1. Example
The host could use this sequence to set up the modem for Protocol 1 SMS:
To set up the modem for Protocol 2 SMS:
In either protocol, the host must wait for the “CONNECT” response before it sends data. Transmission of data
before this message can result in loss of information.
AT:UCA,1,0,4E This sets up the modem for V.23 modulation, Protocol 1, 78
Mark bits in the message header.
AT+FTM=201 Transmit a Protocol 1 SMS frame
<CR><LF>CONNECT <CR><LF> Modem response to the host
AT:UCA,0,12C,4E This sets up the modem for Bell 103 modulation, Protocol 2,
300 bit Channel Seizure, 78 Mark bits in the message header.
AT+FTM=202 Transmit a Protocol 2 SMS frame
<CR><LF>CONNECT <CR><LF> Modem response to the host