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Chapter 5 - Troubleshooting
The MTA128ST-USB disconnects while online
•Check for loose connections between the MTA128ST-USB and the computer, the ISDN jack,
and AC power.
•The problem may have originated at the other end of the line. Try again.
•If you were online with a BBS or online service, it may have hung up on you because of lack
of activity on your part or because you exceeded your time limit for the day. Try again.
The MTA128ST-USB cannot connect when answering
•Autoanswer may be disabled. Turn on autoanswer in your data communications program or
send the command ATS0=1 to your MTA128ST-USB in terminal mode.
Note: If the user wishes to accept calls while DTR is low, the MTA128ST-USB must be configured to ignore
DTR. This is accomplished by entering AT&D0<c r>. With this configuration, the MTA128ST-USB is able
to accept calls while DTR is low. If this configuration setting is not made, the TA rejects incoming calls
until DTR is high while the calls comes in.
File transfer appears slower than it should be
•If you have several active USB devices on the same USB link, try deactivating one device at
a time to see of the transfer rate increases. USB devices on the same USB link share the
same bandwidth of the USB link. There may be several devices using a large amount of
bandwidth (such as a USB removable, media disk drive) leaving a small amount of
bandwidth for the MTA128ST-USB.
Data is being lost
•USB cable is not plugged in.
•Make sure the flow control method you selected in software matches the method selected in
the MTA128ST-USB.
Making a V.120 call, caller gets a NO CARRIER message after dialing the number
Some switches don’t support ISDN SETUP messages that contain a LLC (low layer compatibility)
element. When these switches receive an LLC in the SETUP, they immediately reject the call and a
NO CARRIER message displays. Disable LLC for V.120 calls by issuing the command AT!D0=0. T o
enable LLC for V.120 calls, issue the command AT!D0=1.
Making a V.110 call, caller gets a NO CARRIER message after dialing the number
Some switches don’t support extended BC (bearer capability) information in the ISDN SETUP
message, such as the one normally sent by V.110 data protocol. When these switches receive a BC
with extended capability information in the SETUP, they immediately reject the call and a NO
CARRIER message displays. Disable sending extended BC information for V.110 calls by issuing the
command AT!D1=0. T o enable sending extended BC information for V.110 calls, issue the command
AT!D1=1 (the default setting).
When using X.75 data protocol to transfer data via the Zmodem, the Zmodem displays Intermittent

bad packet

errors and data throughput drops.
This occurs in cases when the X.75 packet size is greater than 1024 bytes and the terminal adapter
sending the file has a fast baud rate (e.g., 115200 bps) or its a USB TA, and the terminal adapter
receiving the file has a slow baud rate (e.g., 19200 bps). To reduce or possibly eliminate this, reduce
the X.75 packet size (S53) to a value less than or equal to 1024 bytes (e.g., ATS53=512). However, if
you set the receiving terminal adapter’s baud rate nearly equal to or greater than the baud rate of the