Silicon Laboratories SI2494/39, SI2493/57/34/15/04 manual Self Test, Loop Test Circuit

AN93

10.2. Self Test

The Si24xx ISOmodem’s advanced design provides the system manufacturer with an enhanced ability to determine system functionality during production tests and to support end-user diagnostics. In addition to local echo, a loopback mode allows increased test coverage of system components. For the loopback test mode, a line- side power source is required. While a standard phone line can be used, the test circuit shown in Figure 45 is adequate.

TIP +

Si3018 VTR

600 Ω

10 μF

IL

>20 mA

RING –

Figure 45. Loop Test Circuit

The AT&Tn command, in conjunction with the AT&Hn command, performs a loopback self test of the modem. AT&Hn determines the modulation used for the test (V.22bis, V.32bis, etc). If an AT&Hn command is not issued just prior to the start of the test, the default or previously-selected modulation is used. The modulation options and defaults are listed in Table 40 on page 77. The test is started with an AT&T2 or AT&T3 command. During the test, the modem is in data mode. To end the test, escape data mode using one of the “Escape” methods, such as +++,

and end the test with ATH.

The AT&T2 command initiates a test loop from the DSP through the DAA interface circuit of the ISOmodem. Transmit data are returned to the DSP through the receive channel. In the parallel or SPI mode, the transmit data are passed to the receiver via Hardware Interface Register 0. AT&T2 tests only the Si24xx chip, not the Si3018/10.

The AT&T3 command initiates a test loop from the DSP through the DAA interface, the ISOcap interface, the Si3018/10, and the hybrid circuit. This test exercises the Si24xx, the Si3018/10, and many external components. A phone line termination with loop current and no dial tone is required for this test since it involves the line-side chip (Si3018/10) and the hybrid. The modem is off-hook during this test. The AT&T3 mode is useful during emitted and conducted radiation testing. To test the ISOcap link only, set U62 (DL) [1] = 1 and issue the AT&T3 command.

The AT\U command is also useful as a production test. This command places a 25 ms low pulse on the RI and DCD pins. It also makes INT the inverse of ESC and RTS the inverse of CTS. Sending the AT\U command can be used to verify the connection of these pins to the circuit board. This command is terminated by resetting the ISOmodem.

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