Silicon Laboratories SI2494/39, SI2493/57/34/15/04 manual Procedure, Example, Crlfconnect Crlf

AN93

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:U708000”.

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:

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
To set up the modem for Protocol 2 SMS:
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

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.

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