Telebit T3000 Reference Manual page 99

&S4 DSR is on only when a carrier is detected from the
remote modem.
& T(n) Test Modes
Range: 0, 1, 3-9
Default:
4
See also: 518, H,
z
Use the &T command to determine if a
communications problem is caused by the local
modem or DTE, the remote modem or DTE, or the
connections between the two sites. When the modem
is performing a test, its Modem Ready (MR)
indicator blinks approximately once per second.
Turn off Error Control before using the &T
command. See the S 180 register.
&T Same as &TO.
&TO Ends the test in progress. To end a test, you can
also use the H (hang-up) or Z (reset) command, or
you can set the S18 register to automatically end
a test after a specific amount of time.
&T1 Checks the path between the local modem and
DTE. Enter & T1, wait for the CONNECT message,
then type a few sentences. If the DTE screen
displays your keyboard input as you type, the local
DTE and modem are operating correctly. End the
test by entering an escape sequence and & TO
command.
&T3 Checks the communication link and the remote
modem. During this test, the local modem sends
incoming data directly back to the remote modem.
Establish a connection with a remote modem, then
enter an escape sequence and & T3 command.
5-24 AT Command Descriptions