Silicon Laboratories SI2494/39 manual Symbol/Data Rate, S41 Symbol Rate Allowable Data Rates, Emi

AN93

The data sent during &T4 and &T5 transmission tests is controlled by the S40 register. The data rate for &T4 and &T5 commands is controlled by the existing &G command. In V.34 cases, where a data rate may use multiple symbol rates, the symbol rate is controlled by the S41 register. If an invalid combination of data/symbol rate is selected, the modem chooses a valid symbol rate. It is the responsibility of the operator to select valid combinations for testing.

Table 153. Symbol/Data Rate

S41	V.34 Symbol Rate	Allowable Data Rates
0 (default)	2400 symbols/second	2400–21600
1	2743 symbols/second	4800–26400
2	2800 symbols/second	4800–26400
3	3000 symbols/second	4800–28800
4	3200 symbols/second	4800–31200
5	3429 symbols/second	7200–33600

After the &T4 or &T5 command is issued and the modulation output has begun, a result code stating CONNECT followed by the data rate (as if the output were an actual connection) is sent. The 300 bps rate does not give the speed after CONNECT. The &G4 command allows V.34/2400bps operation, and &G3 allows V.22bis/1200 bps operation.

The answer tone output must also be measured during homologation testing. A bit in memory allows a continuous answer tone to be output in the same way as a continuous DTMF tone through the AT*Y1 command. After issuing the commands AT&H10 and AT*Y2A, a constant answer tone is produced, and the modem is returned to command mode. Both AT commands need to be sent for each and every tone to be produced. Each of the two commands need to be on its own command line. The tone continues until a character is received or the S7 timer expires. After the command has been terminated, the modem returns on-hook and sends the NO CARRIER message.

For homologation testing, it may be necessary to output the V.29 modulation with transmit data. The +FTM command includes additional codes given in Table 154 to initiate output with the transmit data specified in S40.

Table 154. V.29 Data Rate

+FTM=	Transmit Modulation	Data Rate
53	V.29	7200
55	V.29	9600

The AT+FCLASS=0 command must be sent before any other analoop test or connection is made. The modem must remain on-hook for a time programmed in S-register 50. Any attempt to go off-hook is delayed by this time in 1 s units. S-50 default is 3 seconds.

There is a double CONNECT message for analoop in B1O3, V.21, and V.23 for all options except \V0.

10.4.1. EMI

The ISOmodem chipset and recommended DAA schematic are fully compliant with and pass all international electromagnetic emissions and conducted immunity tests (including FCC parts 15 and 68; EN50082–1). Careful attention to the ISOmodem schematic (page 47), bill of materials (page 48), and layout guidelines ensure compliance with these international standards. In designs with difficult layout constraints, the addition of R12 and R13 to the C8 and C9 recommended capacitors may improve modem performance on emissions and conducted immunity. For such designs, a population option for R12 and R13 may allow additional flexibility for optimization after the printed circuit board has been completed. Also, under some layout conditions, C8 and C9 may improve the immunity to telephone-line transients.

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