private let queue = DispatchQueue(label: "gcdwebserver_queue")

private func setupServer(){

    webServer.delegate = self
    webServer.addDefaultHandler(forMethod: "GET", request: GCDWebServerRequest.self) { (req, completion) in

        if let resp = self.response(for: req) {
            return completion(resp)
        }
    }

    queue.async {
        self.webServer.start(withPort: 8521, bonjourName: "GCD Web Server")
    }

}

private let modbusQueue = DispatchQueue(label: "modbus_queue")
private func initiateTimer() {
    polling()

    timer?.invalidate()
    if #available(OSX 10.12, *) {
        timer = Timer.scheduledTimer(withTimeInterval: pollingInterval, repeats: true) { (_) in
            self.polling()
        }
    } else {
        timer = Timer.scheduledTimer(timeInterval: pollingInterval, target: self, selector: #selector(polling), userInfo: nil, repeats: true)
    }
}

@objc private func polling() {

    for device in self.devices {

        if !device.isInProcess && device.isEnabled {
            self.modbusQueue.async {
                self.connectAndReadValues(device: device)
            }
        }

    }
}

private func connectAndReadValues(device: Device) {

    device.isInProcess = true

    let connect = device.modbus.connect()
    //handling connection status
    //...

    readValues(forDevice: device)
}

private func readValues(forDevice device: Device){
    //some constants

    do {

        let registers = try device.modbus.readRegisters(from: startAddress, count: registersCount)
        device.readState = .success
        device.modbus.close()
        //parse and save data to the app memory just as a dictionary. It saves only one small dictionary per device
        parseRegisters(controllerIP: device.modbus.host, vendor: vendor, registers: registers, startAdd: startAddress)
    } catch let error {
        //handling errors
    }

    //refreshing interface in the main queue
}