As described in Serial Port Signals and Pin Assignments, 9-pin serial ports include six control pins. The following table shows properties associated with the serial port control pins.
Control Pin Properties
DTEs and DCEs often use the CD, DSR, RI, and DTR pins to indicate whether a connection is established between serial port devices. Once the connection is established, you can begin to write or read data.
The following example illustrates how these pins are used when two modems are connected to each other.
This example connects two modems to each other via the same computer, and illustrates how to monitor the communication status for the computer-modem connections, and for the modem-modem connection. The first modem is connected to COM1, while the second modem is connected to COM2.
Create the serial port objects —
After the modems are powered on, the serial port object
created for the first modem, and the serial port object
created for the second modem.
s1 = serial('COM1'); s2 = serial('COM2');
Connect to the devices —
connected to the modems. Because the default value for the
ReadAsyncMode property is
data is asynchronously returned to the input buffers as soon as it
is available from the modems.
Because the default
on, the computer (data terminal) is now
ready to exchange data with the modems. To verify that the modems
(data sets) can communicate with the computer, examine the value of
the Data Set Ready pin using the
s1.Pinstatus ans = CarrierDetect: 'off' ClearToSend: 'on' DataSetReady: 'on' RingIndicator: 'off'
The value of the
DataSetReady field is
both modems were powered on before they were connected to the objects.
Configure properties — Both modems are configured for a baud rate of 2400 bits per second and a carriage return (CR) terminator.
s1.BaudRate = 2400; s1.Terminator = 'CR'; s2.BaudRate = 2400; s2.Terminator = 'CR';
Write and read data — Write the
to the first modem. This command puts the modem "off the hook,"
which is equivalent to manually lifting a phone receiver.
ata command to the second modem.
This command puts the modem in "answer mode," which
forces it to connect to the first modem.
After the two modems negotiate their connection, verify the
connection status by examining the value of the Carrier Detect pin
s1.PinStatus ans = CarrierDetect: 'on' ClearToSend: 'on' DataSetReady: 'on' RingIndicator: 'off'
Verify the modem-modem connection by reading the descriptive message returned by the second modem.
s2.BytesAvailable ans = 25 out = fread(s2,25); char(out)' ans = ata CONNECT 2400/NONE
Now break the connection between the two modems by configuring
off. To verify the modems are disconnected,
examine the Carrier Detect pin value.
s1.DataTerminalReady = 'off'; s1.PinStatus ans = CarrierDetect: 'off' ClearToSend: 'on' DataSetReady: 'on' RingIndicator: 'off'
Disconnect and clean up — Disconnect the objects from the modems and remove the objects from memory and from the MATLAB® workspace.
fclose([s1 s2]) delete([s1 s2]) clear s1 s2
Data flow control or handshaking is a method used for communicating between a DCE and a DTE to prevent data loss during transmission. For example, suppose your computer can receive only a limited amount of data before it must be processed. As this limit is reached, a handshaking signal is transmitted to the DCE to stop sending data. When the computer can accept more data, another handshaking signal is transmitted to the DCE to resume sending data.
If supported by your device, you can control data flow using one of these methods:
Note: Although you might be able to configure your device for both hardware handshaking and software handshaking at the same time, MATLAB does not support this behavior.
To specify the data flow control method, use the
FlowControl property. If
hardware handshaking is used to control data flow. If
software handshaking is used to control data flow. If
no handshaking is used.
Hardware handshaking uses specific serial port pins to control data flow. In most cases, these are the RTS and CTS pins. Hardware handshaking using these pins is described in The RTS and CTS Pins.
the RTS and CTS pins are automatically managed by the DTE and DCE.
To return the CTS pin value, use the
Configure or return the RTS pin value with the
Note: Some devices also use the DTR and DSR pins for handshaking. However, these pins are typically used to indicate that the system is ready for communication, and are not used to control data transmission. In MATLAB, hardware handshaking always uses the RTS and CTS pins.
If your device does not use hardware handshaking in the standard
way, then you might need to manually configure the
In this case, you should configure
RequestToSend value that you specify
might not be honored. Refer to the device documentation to determine
its specific pin behavior.
Software handshaking uses specific ASCII characters to control data flow. These characters, known as Xon and Xoff (or XON and XOFF), are described in the following table.
Software Handshaking Characters
Resume data transmission
Pause data transmission
When using software handshaking, the control characters are sent over the transmission line the same way as regular data. Therefore, only the TD, RD, and GND pins are needed.
The main disadvantage of software handshaking is that Xon or Xoff characters are not writable while numerical data is being written to the device. This is because numerical data might contain a 17 or 19, which makes it impossible to distinguish between the control characters and the data. However, you can write Xon or Xoff while data is being asynchronously read from the device because you are using both the TD and RD pins.
Suppose you want to use software flow control with the example described in Example — Reading Binary Data. To do this, you must configure the oscilloscope and serial port object for software flow control.
fprintf(s,'RS232:SOFTF ON') s.FlowControl = 'software';
To pause data transfer, write the numerical value
To resume data transfer, write the numerical value