Section 3: Modems; Connecting A Modem To The Relay(S); Setting The Remote Modem; Connecting To The Remote Modem - Siemens 7SR21 Manual

Section 3: Modems

The communications interface has been designed to allow data transfer via modems. However, IEC 60870-5-103
defines the data transfer protocol as an 11 bit format of 1 start, 1 stop, 8 data and even parity, which is a mode
most commercial modems do not support. High performance modems will support this mode, but are expensive.
For this reason, a parity setting is provided to allow use of easily available and relatively inexpensive commercial
modems. This will result in a small reduction in data security and the system will not be compatible with true IEC
60870-5-103 control systems.
3.1

Connecting a Modem to the Relay(s)

RS232C defines devices as being either Data Terminal Equipment (DTE) e.g. computers, or Data
Communications Equipment (DCE), e.g. modems, where one is designed to be connected to the other. In this
case, two DCE devices (the modem and the fibre-optic converter) are being connected together, so a null terminal
connector is required, which switches various control lines. The fibre-optic converter is then connected to the
relay Network Tx to Relay Rx and Network Rx to Relay Tx.
3.2

Setting the Remote Modem

The exact settings of the modem are dependent on the type of modem. Although most modems support the basic
Hayes 'AT' command format, different manufacturers use different commands for the same functions. In addition,
some modems use DIP switches to set parameters, others are entirely software configured.
Before applying settings, the modem's factory default settings should be applied, to ensure it is in a known state.
Several factors must be considered to allow remote dialling to the relays. The first is that the modem at the
remote end must be configured as auto answer. This will allow it to initiate communications with the relays. Next,
the user should set the data configuration at the local port, i.e. baud rate and parity, so that communication will be
at the same rate and format as that set on the relay and the error correction is disabled.
Auto-answer usually requires two parameters to be set. The auto-answer setting should be switched on and the
number of rings after which it will answer. The Data Terminal Ready (DTR) settings should be forced on. This tells
the modem that the device connected to it is ready to receive data.
The parameters of the modem's RS232C port are set to match those set on the relay, set 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 the
device may be able to communicate with the modem at say 19200 bps, the modem may only be able to transmit
over the telephone lines at 14400 bps. Therefore, a baud rate setting on which the modem can transmit should be
chosen. In this example, a baud rate of 9600 should be chosen.
As the modems are required to be transparent, simply passing on the data sent from the controller to the device
and vice versa, error correction and buffering is turned off.
If possible, Data Carrier Detect (DCD) should be forced 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.
3.3

Connecting to the Remote Modem

Once the remote modem has been configured correctly, it should be possible to dial up the modem and make
connection to the relay. As the settings on the remote modem are fixed the local modem should negotiate with it
on connection, choosing suitable matching settings. If it cannot do this, the local modem should be set with
settings equivalent to those of the remote modem as described above.
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7SR21 & 7SR22 Data Communications
Chapter 4 Page 12 of 15