232RL as COM port: Recognize the device on commercial product - from the software's perspective

Question

I am making a device that uses 232RL to translate from UART to USB and windows see a COM port. The connections are like so:

^{simulate this circuit – Schematic created using CircuitLab}

I want to find a way so that when the customer plugs the product and opens their software, the software can find in which port the product is connected to. I design both the Hardware (product) and the software, so I can have access to anything I might need to change.

My solution, which I think is not optimal: I solved this problem by opening the COM ports one by one (done automatically from the software from COM port 0 to 255) until I find a COM port that sends a code that only the software and the product knows. That way, I know I opened the correct COM port and continue with the communication. (This takes less than a second so it works for me, I just feel there must be a better way)

The question: Is there a 'faster', simplier way to overcome this issue? Like, Can I program the 232RL so that it shows a specific name (vendor name) along with the COM port?

I use win32 API and C/C++ for the software, but I can switch to another API or library if I need to, I can even change the 232RL to another IC if that is what It takes.

Answer 1

You can list which serial ports exist in the system so you don't have to blindly open all ports that could exist, but only the ports that actually are connected at the moment.

You can also query the description of the serial ports, so you can skip the ports that don't look like FTDI USB ports, and only try opening the ports that look like FTDI USB ports.

And finally skip ports that are already opened by other programs.

FTDI also provides a tool for configuring the FT232RL EEPROM parameters so it can be used to customize some things that might be helpful for detecting the port.

Answer 2

The best way to do that, is to find the PID/VID number of the usb chip for the com port, since it's a "virtual com port" through USB, it still has the USB layer.

You should be able to find that through the device manager -> com port -> properties.

Here is a C# method that allows you to get the COM Port list based on a PID/VID. Note that it will return any connected devices with that PID/VID.

        /// <summary>
        /// Compile an array of COM port names associated with given VID and PID
        /// </summary>
        /// <param name="VID"></param>
        /// <param name="PID"></param>
        /// <returns>List of COM port with matching pid/vid</returns>
        List<string> ComPortNames(String VID, String PID)
        {
            String pattern = String.Format("^VID_{0}.PID_{1}", VID, PID);
            Regex _rx = new Regex(pattern, RegexOptions.IgnoreCase);
            List<string> comports = new List<string>();
            RegistryKey rk1 = Registry.LocalMachine;
            RegistryKey rk2 = rk1.OpenSubKey("SYSTEM\\CurrentControlSet\\Enum");
            foreach (String s3 in rk2.GetSubKeyNames())
            {
                RegistryKey rk3 = rk2.OpenSubKey(s3);
                foreach (String s in rk3.GetSubKeyNames())
                {
                    if (_rx.Match(s).Success)
                    {
                        RegistryKey rk4 = rk3.OpenSubKey(s);
                        foreach (String s2 in rk4.GetSubKeyNames())
                        {
                            RegistryKey rk5 = rk4.OpenSubKey(s2);
                            RegistryKey rk6 = rk5.OpenSubKey("Device Parameters");
                            comports.Add((string)rk6.GetValue("PortName"));
                        }
                    }
                }
            }
            return comports;
        }

The 232RL datasheet states:

USB Vendor ID (VID) 0403h FTDI default VID (hex)

USB Product ID (PID) 6001h FTDI default PID (hex)

Thus be used with ComPortNames("0403", "6001");

That will return a list of devices with that pid/vid, that should be one if you only have one connected.

The datasheet also states that the PID/VID can be modified on the eeprom.

COM Port uses windows default driver and should not require any third party driver as long it uses the USB virtual COM class even if the pid/vid is changed, but it's not the case for the 232RL which uses proprietary driver. If you change the PID/VID, you need to change it in the driver INF file, but you will then need to resign the driver.

VID are assigned by the USB foundation and the fee is expensive. There are some chip manufacturers, like microchip, that can assign you a VID/PID for free if you use their chip.

If you change the PID/VID, be careful as the system automatically searches for driver, if a driver is registered on windows with that pid/vid, it will install the wrong driver and your system may BSOD.