Silicon Laboratories SI2493/57/34/15/04 - Manual (Page 165)

AN93
Rev. 1.3 165
6.7.2. Receiving Modem-On-Hold Requests
If modem-on-hold is enabled via the +PMH=1 command, the Si2493 may be placed on hold by a remote modem.
The maximum time the modem will remain on hold is configured with the +PMHT setting. Possible values of
+PMHT are given in Table 107. Upon receipt of a modem-on-hold request, the Si2493 will indicate +PMHR:
followed by the code corresponding to the timeout granted. The DCD pin will be deasserted while the modem is on
hold, and the CONNECT result code will indicate a return to data mode. A modem disconnect due to a timeout or
failed negotiation will result in a NO CARRIER result code.
6.8. HDLC: Bit Errors on a Noisy Line
Bit errors can occur on an impaired line. The problem lies in determining and ignoring the spurious data resulting
from poor line conditions and recovering valid data. This example illustrates a typical data-corruption problem due
to a noisy line and the method used to analyze it. Here, the modem is an Si2404 configured with the following
initialization string after reset.
AT+ES=6,,8
AT+ESA=0,0,0,,1,0
AT+ITF=0383,0128
AT:U87,010A
AT+MS=V22
AT:U7A,3
The following data stream was received over a noisy line.
0D 0A 43 4F 4E 4E 45 43 54 20 31 32 30 30 0D 0A 19 BE 20 20 19 B1 19 B0 19 B2 30 93
19 B1 19 B2 30 93 19 B1 19 B2 30 93 19 B1 19 B2 30 93 19 B1 19 B2 19 B2 B6 9E F7 46
19 B0 19 B2 29 C6 19 B0 19 B2 FF 98 89 18 19 B0 19 B2 92 6E EF 14 65 19 B0 19 B2 DA
BE C6 07 EA D8 31 C2 05 3C FA C8 86 C4 40 E6 19 A0 CA EA A8 F9 19 B2 8D 00 57 A5 43
29 19 B0 19 B2 05 CB 14 9F 7C 2D 19 B0 19 B2 19 B2 19 BA 0D 0A 4E 4F 20 43 41 52 52
49 45 52 0D 0A
First, the data will be analyzed to point out the occurrence of bit errors and spurious data. Secondly, a simple
algorithm to filter the data will be proposed. Finally, the resulting valid data will be presented.
Table 104 lists an initial analysis of some recurring data patterns.