What are common barriers to establishing connections over RS-232 serial?

Question:

What are common barriers to establishing connections over RS-232 serial?

Answer:

RS-232 serial ports are locked and released by the application that is currently using them, or expects to use the them frequently. There are many possible reasons that will serial ports are locked, but these are the most common:

  • The port is being used by another application running. Keep in mind that some devices as built-in modems, IrDA, Bluetooth, etc interfaces are also mapped as COMx devices and are equivalent to plain RS-232 ports.
  • The port is just "reserved" by the application even if it is not being used.
  • An application ended without releasing the port.

This is a problem that is common to the use of RS-232 serial connections and there is no standard suggestion or rule to avoid it. Descriptions of this error on some websites most often pertain to modem connections and all of them suggest "non-optimum" solutions, such as:

  • Using the Task Manager to closing applications and stopping any processes related with serial communications. The RS-232 serial port on PCs is often 'locked' by applications that run in the background (e.g. fax software sometimes runs in the background) and are not noticed by the user unless using the "Task Manager" (hit "Ctrl-Alt-Del" to do so).
  • Uninstalling programs that continually monitor the serial port. These are often associated to devices such as a PDA.
  • Or simply, re-starting the computer.

FAQ ID : 64001

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