Using Control Pins :: Controlling Instruments Using the Serial Port (Instrument Control Toolbox™)

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Using Control Pins

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Control Pins Signaling the Presence of Connected Devices Controlling the Flow of Data: Handshaking

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Signaling the Presence of Connected Devices

Controlling the Flow of Data: Handshaking

Control Pins

As described in Serial Port Signals and Pin Assignments, 9-pin serial ports include six control pins. The properties associated with the serial port control pins are as follows:

Serial Port Control Pin Properties

Property Name	Description
`DataTerminalReady`	Specify the state of the DTR pin.
`FlowControl`	Specify the data flow control method to use.
`PinStatus`	Indicate the state of the CD, CTS, DSR, and RI pins.
`RequestToSend`	Specify the state of the RTS pin.

Signaling the Presence of Connected Devices

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.

You can monitor the state of the CD, DSR, and RI pins with the PinStatus property. You can specify or monitor the state of the DTR pin with the DataTerminalReady property.

The following example illustrates how these pins are used when two modems are connected to each other.

Example: Connecting Two Modems

This example (shown on a Windows machine) connects two modems to each other via the same computer, and illustrates how you can 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 instrument objects — After the modems are powered on, the serial port object s1 is created for the first modem, and the serial port object s2 is created for the second modem.
```
s1 = serial('COM1');
s2 = serial('COM2');
```
Connect to the instruments — s1 and s2 are connected to the modems. Because the default value for the ReadAsyncMode property is continuous, data is asynchronously returned to the input buffers as soon as it is available from the modems.
```
fopen(s1)
fopen(s2)
```
Because the default value of the DataTerminalReady property is on, the computer (data terminal) is now ready to exchange data with the modems. You can verify that the modems (data sets) are ready to communicate with the computer by examining the value of the Data Set Ready pin using thePinStatus property.
```
s1.Pinstatus
ans = 
    CarrierDetect: 'off'
      ClearToSend: 'on'
     DataSetReady: 'on'
    RingIndicator: 'off'
```
The value of the DataSetReady field is on because 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 atd command to the first modem. This command puts the modem "off the hook," which is equivalent to manually lifting a phone receiver.
```
fprintf(s1,'atd')
```
Write the ata command to the second modem. This command puts the modem in "answer mode," which forces it to connect to the first modem.
```
fprintf(s2,'ata')
```
After the two modems negotiate their connection, you can verify the connection status by examining the value of the Carrier Detect pin using the PinStatus property.
```
s1.PinStatus
ans = 
    CarrierDetect: 'on'
      ClearToSend: 'on'
     DataSetReady: 'on'
    RingIndicator: 'off'
```
You can also 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 the DataTerminalReady property to off. You can verify that the modems are disconnected by examining 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
```

Controlling the Flow of Data: Handshaking

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 may be able to configure your device for both hardware handshaking and software handshaking at the same time, the Instrument Control Toolbox software does not support this behavior.

You can specify the data flow control method with the FlowControl property. If FlowControl is hardware, then hardware handshaking is used to control data flow. If FlowControl is software, then software handshaking is used to control data flow. If FlowControl is none, then no handshaking is used.

Hardware Handshaking

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.

If FlowControl is hardware, then the RTS and CTS pins are automatically managed by the DTE and DCE. You can return the CTS pin value with the PinStatus property. You can configure or return the RTS pin value with the RequestToSend property.

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. For the Instrument Control Toolbox software, 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 RequestToSend property. In this case, you should configure FlowControl to none. If FlowControl is hardware, then the RequestToSend value that you specify might not be honored. Refer to the device documentation to determine its specific pin behavior.

Software Handshaking

Software handshaking uses specific ASCII characters to control data flow. These characters, known as Xon and Xoff (or XON and XOFF), are described below.

Software Handshaking Characters

Character	Integer Value	Description
Xon	17	Resume data transmission.
Xoff	19	Pause data transmission.

When using software handshaking, the control characters are sent over the transmission line the same way as regular data. Therefore you need only the TD, RD, and GND pins.

The main disadvantage of software handshaking is that you cannot write the Xon or Xoff characters while numerical data is being written to the instrument. 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 instrument because you are using both the TD and RD pins.

Example: Using Software Handshaking. Suppose you want to use software flow control in conjunction with your serial port application. To do this, you must configure the instrument and the serial port object for software flow control. For a serial port object s connected to a Tektronix TDS 210 oscilloscope, this configuration is accomplished with the following commands.

fprintf(s,'RS232:SOFTF ON')
s.FlowControl = 'software';

To pause data transfer, you write the numerical value 19 (Xoff) to the instrument.

fwrite(s,19)

To resume data transfer, you write the numerical value 17 (Xon) to the instrument.

fwrite(s,17)

Events and Callbacks

Controlling Instruments Using TCP/IP and UDP

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