public void doPrimaryTcpConnections() {
        try {
            primaryChannel = channelFactory.createChannel(new LoggingHandler(LogLevel.TRACE));
        } catch (Exception ex) {
            primaryChannel = null;
            logger.error("doPrimaryTcpConnections: " + ex.toString());
        }
        if (primaryChannel != null) {
            if (primaryChannel.isActive()) {
                primaryChannel.pipeline().addFirst(MULTIPLEXER, s7hmux);
                ((S7HMux) s7hmux).setPrimaryChannel(primaryChannel);
            }
        }
    }
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