Silicon Laboratories SI2493/57/34/15/04 - Manual (Page 238)

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238 Rev. 1.3
After the host receives “CONNECT “, it sends the frame data followed by <DLE><ETX>. A frame includes type,
length, payload and checksum. The frame data can contain anything, including <DLE><ETX>. No DLE shielding
is needed. <DLE><ETX> with no data for about 8ms is treated as the end of frame.
After a frame has been sent, the modem response will be: <CR><LF>OK<CR><LF>
On the answer modem side, the command AT+FRM=200 causes the receiver to look for at least 32 bits of CS and
for at least 60 bits of marks. The answer modem detects the
protocol of the transaction by whether CS comes
before the marks.
There can be several responses to AT+FRM=200, depending on the received data.
9.2.2.2. Response 1
If the frame had not been received within the time specified in RxTimeout, the modem response to the host would
be:
<DLE><ETX><CR><LF>NO CARRIER<CR><LF>
9.2.2.3. Response 2
If the frame had been received with a mark segment at beginning of frame, the modem would respond to the host
with:
CONNECT 1<CR><LF>
<Frame Data Received><DLE><ETX>
<CR><LF>OK <CR><LF>
No frame checking would be done by the modem; all of that would be left to the host. As shown above,
<DLE><ETX> marks the end of frame. After sending OK to the host, the modem goes back into command mode.
9.2.2.4. Response 3
If the frame had been received with channel seizure and mark segment detected at beginning of frame, the modem
would send one of the responses below to the host:
CONNECT 2<CR><LF>
<Frame Data Received><DLE><ETX>
<CR><LF>OK <CR><LF>
The Host does all upper level frame validations, since the modem does not do any frame checking. As shown
above, <DLE><ETX> signifies the end of frame. After the modem sends OK, it goes back to command mode.
AT+FRM=200 Receive an SMS frame
AT:UD2,<RxTimeout> Set RxTimeout: