7SG12 DAD-N Communication Interface
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4.2 Setting the Remote Modem
Most modems support the basic Hayes ‘AT’ command format, though different manufacturers can use
different commands for the same functions. In addition, some modems use DIP switches to set parameters
while others are entirely software configured. Before applying the following settings it is necessary to
return the modem to its factory default settings to ensure that it is in a known state.
The remote modem must be configured as Auto Answer, which will allow it to initiate communications with
the relays. Auto answer usually requires 2 parameters to be set. One switches auto answer on and the
other, the number of rings after which it will answer. The Data Terminal Ready (DTR) settings should be
forced on which tells the modem that the device connected to it is ready to receive data. The parameters
of the modem’s RS232C port need to be set to match those set on the relay i.e. baud rate and parity to be
the same as the settings on the relay, and number of data bits to be 8 and stop bits 1.
Note: although it may be possible to communicate with the modem at e.g. 19200bps, it may not be
possible to transmit at this rate over the telephone system, which may be limited to 14400. A baud rate
setting needs to be chosen which is compatible with the telephone system. As 14400 is not available in the
relay, the next lowest rate, 9600, would have to be used.
Since the modem needs to be transparent, simply passing on the data sent from the controller to the
device and vice versa, the error correction and buffering must be turned off. In addition if possible force the
Data Carrier Detect (DCD) setting to ON as this control line will be used by the fibre-optic converter.
Finally these settings should be stored in the modem’s memory for power on defaults.
4.3 Connecting to the remote modem
Once the remote modem is configured correctly it should be possible to dial into it using the standard
configuration from a local PC. As the settings on the remote modem are fixed, the local modem should
negotiate with it on connecting and choose suitable matching settings. If it does not, however, set the local
modem to mimic the settings of the remote modem described above.
©2010 Siemens Protection Devices Limited
Chapter 4 Page 5 of 10
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