Specifications

54
Schneider Electric
Interfaces (continued)
Modem parameters:
b Error correction can be used to eliminate a number of transmission errors on the
often disturbed telephone network. ITU standard V.42 is faster and better than the
MNP-4 (from Microcom).
b Data compression can be used to accelerate the transfer of data that has not yet
been compressed. With ITU standard V.42bis, the data compression rate can be as
high as 4:1, and compressed data is recognized. MNP-5 (from Microcom) provides
a compression rate of only 2:1.
b Flow control: slow modems may have only software flow control, known as XON/
XOFF in manuals, but faster modems must use hardware flow control, with RTS/CTS
control lines. Use of both modes, possible with some modems, is not recommended.
Common AT commands:
Note: The modem reference manual is the most reliable source of information.
Examples given here may differ.
Messages:
ATD To dial a number, followed by a T for frequency selection (DTMF) or, more
rarely, D for pulses (for older telephone switching stations). Example: ATDT
6939332 [ENTER] +++ can be used to return to the control mode and, for
example, hang up with ATH. Remember to pause for one second before and
after the command.
AT&W Records the profile currently in memory. This is the profile that will be
activated when the modem is turned on or with the ATZ command. Note that
several profiles can be recorded on some models.
AT&V Used to view active modem parameters.
AT&F Resets all modem parameters to their original factory settings.
ATLn Changes speaker volume. (ATMn selects speaker operating mode.)
ATSr=n Places value n in register r. The command ATSr can be used to view the
contents of the register.
ATS0=n Sets the number of rings before the modem answers. If the value is 0 the
modem will not respond. In Switzerland there must be at least two rings
before the modem answers (according to OFCOM).
ATS7=n Used to define the time the modem waits for a carrier before hanging up and
indicating NO CARRIER. This value is usually between 45 and 60 seconds.
OK The command has been understood.
ERROR All or part of the command is erroneous.
CONNECT The link is established. This message is often accompanied by other
information on speed and protocol (error correction and/or compression).
NO DIALTONE The modem cannot dial the number because there is no dial tone.
NO CARRIER The carrier has been lost, or the remote modem has hung up, or our modem
did not receive a carrier when calling (for example, when someone picks up
the phone and says "hello!").
NO ANSWER No one is answering, the number is wrong, or the equipment at the other end
has broken down. (This does actually occur.
BUSY All lines are occupied; the modem must try again later or use another number.
Guide to Integration
Modbus / Jbus
For some applications it is useful to know
the modem parameters and how to set
them.
Commands beginning with AT (Hayes
compatible) differ according to the modem.
Modems that use Rockwell chips all have
the same commands. Thus it is best to
describe functions rather than specific
commands.
Modem parameters are:
b Error correction,
b Data compression,
b Flow control.