Verifying the DTE-to-Modem Connection
The first interface to verify is the one between the DTE and the modem.
This involves the cable, the modem, the software, and the DTE.
Cable Requirements
When a cable is used to carry signals between the DTE and the modem
(external modem) any cable with transmit, receive, and ground will support
standard asynchronous communications. To make sure that communications are
performed properly, all other transmission modes require additional signals.
For example, synchronous communication requires timing signals. Error-control
communication with hardware local flow control requires RTS and CTS signals.
To determine the signal capabilities of a cable, you can issue the &T19
command (supported only by V-series products). The &T19 command is discussed
later in this section.
Another cabling factor that can affect communications is shielding. If any
of the cables that are connected to the DTE (not just the one connecting
the modem to the computer) are not shielded, interference may occur causing
data errors. In all cases, the connector and the cable wires should be shielded
to reduce the possibility of interference.
DTE Port or Slot
Not every computer has a serial port (the kind necessary for modem communications).
Also, that port may already be used by a mouse. Some computers support a
parallel printer. If the modem is an internal modem, and it has been installed
correctly, this should not be a problem. However, selection of the port
is important, no matter whether it is an internal or external modem. Improper
port selection is a frequently diagnosed problems.
Command Processor of the Modem and the Software
The following process helps troubleshoot an unfamiliar modem/software
combination. This could be either for a new program or the addition of a
new modem to an existing installation. The process tests asynchronous operation.
However, if troubleshooting a synchronous connection, you need to verify
the asynchronous portion of the modem operation anyway. The following steps
verify that command information is getting from the DTE to the modem and
that its command processor is operating properly.
Process
If you are using communications software (and not a terminal), consult
the user guide for suggestions and the proper procedure for issuing commands
directly to the modem. Some software does not permit direct command interface
with the modem. It is essential that you be able to establish a direct connection
between the DTE and the modem, so that what you type is transmitted through
the computer directly to the modem.
Step 1:
Try issuing AT<CR>. The OK result code should be returned by the
modem. The modem should be receiving your commands and be responding with
a result code. If commands do not seem to be reaching the modem, check the
cable connection. Make sure you are addressing the desired communication
port (COM1 or COM2). This option is set either with a software option or
with an operating system command. For example, use the Chooser to select
a COM port for the Macintosh. The DOS SET command is used for IBM PCs and
compatibles. The physical connection of the modem to a terminal determines
the port.
Step 2:
If you can not see anything you type, or if for any other reason, you
suspect the modem settings may be preventing the connection, issue AT&F<CR>to
restore the modem factory configuration. If the modem is a Vseries product,
try issuing AT&Q0<CR>to put it into normal asynchronous mode.
In this mode, any software should talk to the modem. For example, local
flow control is not an issue if it is configured for standard asynchronous
mode.
Step 3:
Try issuing commands to the modem. For example, try the I commands used
to identify the modem. If OK or the expected result codes (see the I
command) are returned, the modem is operating as it should. Try changing
a few configuration settings, then resetting the modem with ATZ<CR>.
If OK is returned, this part of the modem command processor is functioning
properly.
If no responses are returned from the modem, try issuing ATE1<CR>to
enable command state echo. If characters are double, issue ATE0<CR>to
disable command state echo, as the software is providing that echo already.
Then try issuing some commands or reading some S-registers.
Analysis
If you are using different communications software before running this
procedure, either that software is not compatible with the modem, or an
incorrect setting (e.g., communication port) that prevented the connection
was corrected with the software used to make the direct connection. This
setting should be changed in the original software.
If the connection was made with the same software that previously exhibited
a problem, but no problem occurred when in direct connect mode, check the
settings used in the command-oriented process above to make sure that the
standard software interface (probably menu-driven) is storing settings such
as COM port selection, speed, and character format. This may all seem too
elementary, but a simple conflict between modem and software settings typically
causes the problem.
If the process works in asynchronous mode, but does not in error-control
or ASB mode, you may have cabling and/or local flow control problems.
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