UM-0085-B09
DT80 Range User Manual
Page 201
RG
Modem Initialisation
Configuring Your Modem
Modems are highly configurable devices with many different parameters that can be adjusted. To make this long list of
parameters easier to manage, modems normally support a number of different configuration profiles. Each profile
consists of a complete set of parameter values. As a minimum, there would be a fixed factory settings profile, and a user-
configurable default profile. The default profile's settings are automatically loaded when the modem is reset or powered
up.
Although the DT80 includes support for automatically sending initialisation commands to the modem, it is recommended
that you fully pre-configure the modem, and write the configuration to the modem's default profile, before connecting it to
the DT80. This ensures that whenever the modem resets due to power loss or other reason it will start with the correct
configuration.
Sending Modem Commands
Refer to your modem's user's manual for information on how to go about pre-configuring the modem. This is usually
done by connecting the modem to a PC serial port and running a terminal program such as Hyperterminal or
DeTransfer
so that you can then send "AT" commands to configure the modem.
You can also use the SSDIRECT command (see
Serial Sensor Direct Mode (P356)
) to configure a modem that is
already connected to the host port of the DT80. To use this feature, connect to the DT80 using the USB or Ethernet port,
then send the command
SSDIRECT 2
to enable Serial Sensor Direct Mode. You can now interact with a modem attached to the host RS232 port. for example,
you can sent AT commands and see its responses (assuming the modem is in command mode). When you're done,
enter
ENDSSDIRECT
Recommended Option Settings
The following is a list of items that should be set in the modem's default profile. You should check your modem user's
manual to determine the exact command to use for each setting.
It is recommended that you start by resetting the modem to its factory defaults so that you have a known starting point.
This is particularly important if someone else has used the modem before and you are uncertain of the settings used.
For most modems the
AT&F
command will load the factory default profile. After this command has been issued, you can
then set all other settings as detailed below and as required for your specific application.
•
DSR always on
: The
DSR
(data set ready) signal from the modem must be active while the modem is turned
on, as this is how the logger determines that a modem is connected. Most modems do this by default, if not then
you must set the modem do this (typically by using the command
AT&S0
). If the modem cannot be configured
to operate in this way then the logger must be tricked by connecting
DSR
to
DTR
at the logger side, but this
should be done as a last resort as it is not as robust a method. It is important for the logger to know that a
modem is connected so that it can avoid sending normal data output to the modem when the modem is in
command mode.
•
DCD follows carrier state
: The
DCD
(data carrier detect) signal must be set to follow the carrier state, as this
is how the logger determines when a connection has been established with another modem. Most modems do
this by default, if not then you must set the modem to do this (typically using the command
AT&C1
).
•
Hardware flow control
: We recommend that you always use hardware flow control on the modem and the
logger. For most modems this is the default. It is typically set using the
AT&K3
command.
•
Force reliable (error correcting protocol) mode
: You should ensure that the modem connection always uses
an error correcting protocol such as LAPM or MNP. This is required to ensure the flow control information is
passed between the logger and PC based application as well as to eliminate line errors that may corrupt the
data transmission. If an error correcting protocol is not used then line errors and flow control problems can
cause data loss. Many modems will fall back to a non error correcting mode if an error correcting protocol
cannot be agreed with the other modem. This should be avoided as it can cause data loss. Many PSTN
modems can be set to force reliable mode using the
AT\N2
command. That is, they will not connect at all if an
error correcting protocol is unavailable.
•
Auto answer
: The DT80 does not issue any commands to the modem to answer a call. Therefore, if dial-in
functionality is required, the modem must be set to auto-answer incoming calls. For most modems the
command
ATS0=4
will set the modem to auto-answer incoming calls after 4 rings. This setting may be omitted
if you do not need dial-in functionality, in which case the modem will ignore incoming calls.
•
Don’t echo commands
: Most modems will echo incomming commands. This is not desirable when used with a
DT80 as the echo may be treated as a command by the logger which will only confuse it. For most modems the
command
ATE0
will turn off echo of modem commands.
•
Quiet mode
: Most modems will issue messages whenever a command is sent to the modem or a connection is
made or dropped. This is not desirable when used with a DT80 as the message may be treated as a command
by the logger which will only confuse it. For most modems the command
ATQ1
will disable the output of such
messages.
