6.5.1 Using multiple meters with Westermo TD-36 PSTN modem and RS-485
The Westermo TD-36 is an example of an industry PSTN (Public Switched Telephone Network)
modem with both RS-485 and RS-232 connections. The RS-485 is an easy solution if you want to
connect one or more meters to one modem/telephone line. Always choose an industry class modem
rather than office equipment when building communications solutions for your meters in substations.
The environment is normally quite different from an office, both regarding physical environment
(like temperature) and EMC environment.
The figure below shows a meter connected to the modem using RS-485 (there are extra connectors
on the RS-485 cable for multiple meter connection).
Figure 41 - TD-36 modem with RS-485 connection to a meter
When configuring the modem you do not need to do any settings in the modem, just follow the
instructions below and set the DIP-switches as shown in the red markings below.
Also make sure that the meter is configured for RS-485 (see 6.5.1.1).
The cable connecting the modem and the meter may be equipped with a termination resistor of 120
ohms.
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