Silicon Laboratories SI2493/57/34/15/04 - Manual (Page 224)

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8. Security Protocols
The Si24xx ISOmodem can handle a variety of security protocols. Two are specifically described here. The
"Ademco® Contact ID Protocol" and the SIA protocol.
8.1. Implementing the SIA Protocol
The SIA protocol defines communication between an alarm panel and a central station. In a traditional security
system, the alarm panel always calls the central station and sends data; the central station only acknowledges its
readiness to receive data and that data has been received. For this reason, the communication part of the alarm
panel has been called the transmitter; the communication part of the central station has been called the receiver.
With the SIA protocol, the central station can also send data to the alarm panel; since data is sent using FSK, the
communication can be half-duplex or full-duplex. Nevertheless, the traditional nomenclature of transmitter for the
alarm panel and receiver for the central station is still used for the SIA protocol.
The communication session consists of four elements:
 The Handshake Tone (a single tone)
 The Speed Synchronization Signal (two tones)
 Data Blocks and Control Signals (transmitted using Bell 103 FSK encoded data)
 Acknowledgement Blocks (can be either single tones or Bell 103 FSK encoded data, according to the
capabilities of the transmitter)
8.1.1. Modem-Specific Implementation Details
8.1.1.1. Listen-In and V-channel Periods (Voice Pass-Through)
This can be accomplished with the following procedure using standard voice modem commands:
1. Prior to making the call, the host issues AT+VNH=2 to modem. This will keep the modem off-hook when the
host clears down the FSK data connection using ATH.
2. After the modem has negotiated a Listen-in period using the SIA protocol in Bell 103, the host clears down the
FSK data link with ATH.
3. The host issues AT+VNH=2 to the modem again to maintain off-hook status for next cleardown.
4. The host then issues AT+FCLASS=8 followed by the usual Si3000 voice pass-through command sequence. (To
minimize the transition time, the host should set up the Si3000, TX/RX voice filters and all gain stages before
the call.)
5. AT+VLS=5 starts the Si3000 pass through, which supports Listen-in as well as V-Channel (bi-directional voice)
operation. Use +VLS=13, +VSP=1 etc. if speakerphone is desired.
6. When voice period is over, the host puts the modem back in data mode using AT+FCLASS=0. This will
terminate voice operation as well as going to +FCLASS=0. ATH and +VLS=0 must not be used in +FCLASS=8
to terminate the voice session because they override +VNH=2 (per V.253 standard).
7. The host again issues AT+VNH=2 to the modem to keep it off-hook for the next cleardown.
8. AT+F0 puts the transmitter in Bell 103, SIA mode to resume SIA protocol communication.
8.1.1.2. Inserting a V.32bis period (e.g., SIA Level-3 Video Block Support)
This can be accomplished with the following procedure using standard voice modem commands:
1. After clearing down the SIA FSK link, the host issues AT+VNH=2 to the modem to maintain off-hook status for
the next cleardown.
2. The host sets the modem to V.32bis; i.e., AT&H4, AT\N3 (if error correction is desired)
3. The host selects V.32bis originate or answer mode: AT%O2 if the transmitter should assume the handshake
mode of an originating V.32bis modem; AT%O1 if it should assume the handshake mode of an answering
modem.