public void doSecondaryTcpConnections() {
        try {
            secondaryChannel = secondaryChannelFactory.createChannel(new LoggingHandler(LogLevel.TRACE));
        } catch (Exception ex) {
            secondaryChannel = null;
            logger.info("doSecondaryTcpConnections(): " + ex.toString());
        }
        if (secondaryChannel != null) {
            if (secondaryChannel.isActive()) {
                secondaryChannel.pipeline().addFirst(MULTIPLEXER, s7hmux);
                ((S7HMux) s7hmux).setSecondaryChannel(secondaryChannel);
            }
        }
    }
Subscriptions are only supported in a small subset of the S7 devices.

@return true, if the device supports subscriptions.
plc4j/drivers/s7/src/main/java/org/apache/plc4x/java/s7/readwrite/protocol/S7HPlcConnection.java