8 IP Modem Function
8 IP Modem Function
The Firmware offers the function of IP Modem. Used in this mode, the serial port of the NetCom
+
emulates a standard serial modem. Basically this means the NetCom
+
will
a) answer to AT-commands on the serial port
b) establish a connection to a destination
c) inform the connected serial device of the connection
d) accept a TCP connection, and inform the serial device of that event
For connections the NetCom
+
will use a TCP connection. This differs from a normal telephone
line, so there will be some modifications in the behavior. The target is an IP Address, not a phone
number. Also for hardware reasons the automatic baudrate detection used in today serial Modems
is not available. However this is not a problem at all, the IP Modem can be installed in Windows
as a Standard 33600bps Modem. There is an INF-file for ease of installation.
8.1 Some possible Scenarios
1. The customer has a remote management installation, operating via telephone line. These
lines may be in-house or through public phone systems to other destinations. The customer
wants to reduce costs for these lines, management and possibly hardware, using the Intra- or
Internet.
2. The customer wants to contact several stations from a central server. Because of frequent
target changes he does not want to define the target by a Virtual Com Port.
3. Remotely distributed devices contact a central system by Modem. This is the reverse of
option 1.
4. A computer without Network access shall have at least limited control on the connections
established by a NetCom
+
.
5. Old fashioned BBS installations become accessible via Internet. The typical multi-modem
box is replaced by a NetCom
+
Server with multiple ports configured for IP Modem operation.
8.2 Serial Signals and Cables
A real modem provides the same signals as the serial port of a PC. However, where a signal is an
output on the PC, it is an input to the modem, and vice versa. So in the NetCom
+
the emulation
of a modem must be incomplete. By exchanging RxD and TxD the data connection is fine, the
same happens for handshaking signals RTS and CTS. The DTR of the PC is connected to DSR of
NetCom
+
, this is simple. The RI may be ignored, some connectors for serial ports also do that.
However a real modem provides DSR and DCD to the PC. There is only the DTR left on NetCom
+
to serve these signals. In most configurations the NetCom
+
-DTR serves as the DCD to the com-
puter.
The cable must provide a DSR to the PC then, e.g. by shortcut to the PC-DTR. In some configu-
rations the NetCom
+
-DTR must serve as the DSR. This is configurable by a command.
September 2016
NetCom Plus User Manual
96
Summary of Contents for NetCom Plus 111
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