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PORTABLE USB MODEM
B.2 Local Analog Loopback Test (V.54 Loop 3)
In this test, data from your computer or terminal is sent to your modem’s
transmitter, converted into analog form, looped back to the modem’s receiver,
converted into digital form, and then sent to your monitor for verification. No
connection to the phone line is required.
Figure B-1. Local analog loopback test.
T
EST
P
ROCEDURE
1. Connect the modem to your computer. Using your communication program,
set the desired baud rate and go into terminal mode.
2. Type AT&T1 and press ENTER. This places your modem in analog loopback
mode in the originate mode. A CONNECT message should appear on your
display. The modem is now out of command mode and in a pseudo-online
mode.
3. Note that the CD LED is on. If you are set for 14,400 bps or higher, a speed
LED should be on. If the CD LED is not on, there is a defect in your modem.
4. Enter characters from your keyboard. For this test, typing multiple uppercase
U characters is a good way to send an alternating test pattern of binary ones
and zeros. The characters entered should be displayed on your monitor. The
TD and RD LEDs should flash when a character is entered.
5. To exit the test, type the escape sequence +++AT and press ENTER. This puts
the modem in online command mode. Then type either AT&T or ATH to
return to command mode.
Computer or Terminal Local MultiModem
Digital Analog
AT&T1
CONNECT
UUUUUUUUU
UUU
Portable USB Modem