User manual

Section 4

AT COMMAND SET

4-21

AT&T TEST Modes

Parameters : 0 to 6

Default : 0 (No Tests in Progress)

This command enables and disables several local and remote loopback tests, and tests with

the internal test pattern generator.

AT&T0 - Terminate test in progress.

AT&T1 - Initiate Local Analog Loopback.

AT&T4 - Enable Remote Digital Loopback response.

AT&T5 - Disable Remote Digital Loopback response.

AT&T6 - Initiate Remote Digital Loopback

AT&U USER Flow Control

Parameters : 0 to 3

Default : 0 (Flow Control off)

This command affects flow control in the modem. AT&Un is only active while MNP /V42

is disabled and while the Constant Speed Interface is enabled. If MNP/V42 is enabled then

AT&U can remain at the defaults. Refer to the Data Transfer Section (Section 6) for more

details. The commands AT&En, AT&Kn, AT&In and AT&Un all influence flow control and

must be correctly set for your application.

Whenever AT&Un is invoked, Xon/Xoff characters may be inserted in the data stream and

corrupt a binary file transfer. Alternately, an Xoff character encountered in a binary file

transfer will halt the modem. Select AT&U0 (off) for binary file transfers.

AT&U0 - Pass through off, line flow control off.

AT&U1 - Pass through enabled, line flow control off.

If Xon/Xoff flow control is active (AT&K1 or AT&K3), Xon/Xoff characters

through from the DTE to the line side buffers of the modem - after being acted

upon.

AT&U2 - Pass through off, line flow control on.

This selection is only valid if RTS/CTS handshaking (AT&K2) is selected. In

this mode, any flow control characters detected in the data received from line

will be acted upon by the line buffer but not passed to the DTE buffer.

AT&U3 - pass through on, line flow control on.

This selection is usually used with DTE flow control set to RTS/CTS, AT&K2.

Flow control characters received at the line buffer will be acted upon and then

passed to the DTE via the DTE buffer. The DTE will be operating in a mode

that enables it to respond to both RTS/CTS and Xon/off flow control. In this

way both the local modem and the remote DTE may flow control the far modem

and DTE.