Reading Xon-Xoff Standard Protocol

Question

I'm trying to read the state of a old printer connected by RS232. In the manual of the printer I found the settings of the port: Baud 9600, 8 bit, 1 stop bit, No Parity, Flux Control Hardware. The printer accepts a text message and I send is something like: <MESSAGE> and for one label works fine.

The problem is the multiprint. If I make a for cycle for print multi messages, I should wait the machine is ready for next print. Still in the manual, I find that the printer uses the Xon-Xoff protocol. Now my cycle should wait for the char (DC1, 11 hex, 17 dec.) XOn for sending the next one message. I haven't find anything about how to read this value from the serial port. Any suggestions?

I have tried this code only for try to understand how I have to set the parameters, but I have one exception when I try to Open the serialport:

        Dim ports As String() = SerialPort.GetPortNames()
        Dim port As String
        For Each port In ports
            If port = "COM1" Then
                '  serialport1.BaudRate = 9600
                '  serialport1.DataBits = 8
                '  serialport1.Parity = Parity.None
                '  serialport1.StopBits = StopBits.One
                '  serialport1.Handshake = Handshake.XOnXOff
                MessageBox.Show(port)
                If SerialPort1.IsOpen = False Then SerialPort1.Open()
                AddHandler SerialPort1.DataReceived, AddressOf DataReceivedHandler
            End If
        Next port