Three ways to send and receive data from Serial devices in Linux

Identify the serial device

There are a few ways to find the serial adapter connected to the computer.

For USB serial devices like our PL2303-DB9 USB Serial Adapter, connect the adapter to the computer, then open a terminal and run:

dmesg | tail

Look for output like:

[54221.132226] usb 1-2.2: pl2303 converter now attached to ttyUSB0

This indicates the serial adpater can be accessed at /dev/ttyUSB0

Alternatively, you can run the following command to print out the newest created serial adapter:

ls -tr1 /dev/tty* | tail -n 1

This will output something similar to:

/dev/ttyUSB0

Screen

Screen is a full-screen window manager capable of connecting to serial devices.

To connect to the serial device at 9600 baud (one of the more common buad rates) run the following command in the terminal:

screen /dev/ttyUSB0 9600

From here you can type text into the terminal to transmit to the connected device, or read output from the device.

Screen commands consist of "C-a" or ctrl+a (holding down the control key, and then pressing the 'a' key, then releasing both), to enter the command input mode, then commands may be entered with another key stroke.

To quit screen and close connection: ctrl+a, k. Then 'y' when prompted to kill the session.

Minicom

Minicom is a serial communication program that has a more user-friendly interface than screen, however with more options comes more choices.

To connect to the serial device at 9600 baud with minicom, from the terminal:

minicom -b 9600 -D /dev/ttyUSB0

Minicom will provide more output when connected, in my case:

Welcome to minicom 2.7.1

OPTIONS: I18n 
Compiled on Jul 22 2021, 00:00:00.
Port /dev/ttyUSB0, 10:42:24

Press CTRL-A Z for help on special keys

From here you can type into the terminal to send to connected serial device.

To quit, similar to Screen, the commands are accepted through key commands. Ctrl+A, z, x will disconnect and close the application.

Minicom may store presets and can be configured using the setup mode, but it is not necessary for basic useage.

minicom -s

Redirecting echo

We can directly write to the serial device from bash by redirecting the output of the 'echo' command to the serial device. First we need to set the serial port speed and then send the text "helloworld" to the serial device:

stty -F /dev/ttyUSB0 speed 9600
echo helloworld >/dev/ttyUSB0

This method will not print responses from the serial device, and is most useful for one-way communication with a device, for example sending a string of text to an LED matrix display.

Troubleshooting Connectivity

Permission denied

Most permission denied error messages can be resolved by adding your user account to the 'dialout' group or the group with control of the serial port. We can check the group ownership of the port with the following:

ls -l /dev/ttyUSB0

crw-rw----. 1 root dialout 188, 0 Jan 21 11:05 /dev/ttyUSB0

In this case I would need to add my user account to the 'dialout' group:

sudo usermod -a $(whoami) -G dialout

`$(whoami)` can be replaced with your user name. Once added to the group, close and reopen the terminal, log out and back in, or restart the computer for the changes to take effect.

Scrambled Text

This is most often caused by incorrect baud rate settings on either the computer or device. Most serial devices like network routers and switches will have either a sticker with the serial port settings, or these can be found in the user manual for the device. For device with additional settings minicom may prove to be the easiest to configure.

Originally authored by: David Roberts, December 9, 2013

The Plugable USB serial adapter allows you to connect legacy devices that use RS232 serial communications to computers that do not have a serial port. Although this usually works well, a number of problems can crop up because many of the devices people want to connect are older devices with various issues. A frequent problem is that the software that is necessary to connect to the device does not work under modern operating systems such as Windows 7 or above. Other problems can include using an improper cable, incorrect port settings, or a basic incompatibility between the Plugable USB serial adapter and the device it is being connected to.

Drivers don’t load correctly, or the cable cannot be seen in Device Manager

When the cable is plugged into Windows 7 or higher computers, the necessary driver should load automatically. If it does not load, make sure the computer is connected to the internet. If an internet connection is not available, download the latest drivers for the cable from the Plugable driver site here. For earlier Windows XP and Vista computers, the drivers must be downloaded from the Plugable website and installed. 

The drivers load successfully, but I can’t communicate with my device

Serial communications over RS232 is a legacy from an earlier age in computer when devices were not necessarily plug-and-play. Several different variables come into play, and all must be set up correctly to ensure communications.

Software

Many legacy devices came with specialized software that is used to communicate with them over a RS232 serial port. Many of these programs were written years ago, and are not as automated as modern applications. In many cases, the software is not compatible with modern Windows operating systems. If the software won’t run, or runs with errors, check the website of the device maker to see what Windows version it is compatible with. Many times you will see a warning notice saying that the software does not work with certain versions of Windows and no update is anticipated. For example, Nikon digital cameras from the late Nineties used a serial connection to transfer photos to a computer. However, this software does not support Windows Vista or later, and Nikon warns on its website that no update is anticipated. 

Some devices don’t come with special software. Instead you communicate with them using a standard serial communications program. A popular and easy to use program is Putty, which can be downloaded for free from the internet. It is also possible to send commands to the COM port directly from the Command Prompt, but this is for experts only. 

COM Port

In Windows, serial communications proceed through a COM port that is assigned by Windows when the USB serial adapter is plugged in. You can find out this COM port by opening Device Manager, going to Ports (COM & LTPT) and looking for Prolific USB-to_Serial Comm Port (COM#). The # symbol represents the port number of the COM port assigned by Windows. This port must be set correctly in whatever application is trying to communicate through that port. 

Because older Windows computers only offered COM ports 1 through 4, many legacy applications require the COM port to be set within this range. Some applications only allow ports 1 or 2 to be set. If the port is out of the range required by the application, you can change the port number by  right-clicking on the adapter in Device Manager, selecting Properties, selecting the Port Settings tab, then clicking Advanced. Clicking on the COM Port Number button reveals a list of ports, with unavailable ones listed as “in use.” Set the port you want here, then click OK to close the Properties dialog. Device Manager should update and show the number you selected. If it does not, make sure you have sufficient permissions to change the COM port settings. 

Port Settings

There are many other settings that must be the same for the port and the device being communicated with. Typically the manual for the device being connected will outline the correct settings. Sometimes the application that is used with the device will allow you to change the settings required by the device.
Generally, however, the settings for the port are changed to match the settings required by the device. These settings are changed by right-clicking on the adapter in Device Manager, selecting Properties and selecting the Port Settings tab. The default settings are appropriate for many devices. However, in many cases the Bits per second, or “baud rate” can be set higher for faster communications. Data bits, Parity, and Stop bits change the format of the data sent to the device. Typically, if these are set wrong, the device will not be able to understand anything sent to it. Flow control selects a hardware (requires a special cable) or software method for controlling communications between the devices that keeps their communications synchronized. Most devices don’t use flow control, and for them, this can be set to Off. 

Cable type

Historically, COM ports were used for serial communications with modems, that took the digital information and modulated it for transmission over telephone lines or radio. Information sent from one modem went to the receiver on the other modem, and was forwarded to the receiving pin on the COM port of the destination computer. Because this “crossover” function was performed by the modems, the cable from the computer to the modem was straight cable, connecting the send pin on the source computer to the send pin on the modem. 

However, when two computers were directly connected to each other, without the intervening modems that connected send to receive, a special cable, called a “null modem” or “crossover” was used that connected the send (Xmit or TX) pin on the source computer to the receive (RECV or RX) pin on the destination computer. 

Which cable to use depends on how the connected device sees itself in relation to the computer. Typically devices that actually do something will require a Null Modem cable, while devices that process data and send it somewhere else (like a modem) will use a straight cable. It’s always good to get the make and model of the device being connected and go online to find out which cable is needed. Many devices come with a cable, which should be used if it is available

If the software is compatible, the port settings are made correctly, and the right cable is used, there are still some cases where the device does not work well with a particular USB Serial adapter. Some manufacturers advise against USB serial adapters alltogether or their website or recommend using a particular chipset. The major alternative available is the FTDI chipset. It is more expensive and works in some situations where the Prolific chipset used in the Plugable serial adapter will not work. Check the manufacturer’s documentation and online forums for information. You can google “<product make and model> USB serial adapter” (without the quotes). Often you’ll find a forum post by someone who has tried using a USB serial adapter and has discovered what works.

Originally authored by: Bernie Thompson, July 5, 2011

Windows 7 will detect and pop up a wizard to install drivers when our Plugable USB to Serial adapter is plugged in, but it’s also easy (as essential on other operating systems) to install the driver software first.  Here’s how.

Get the driver from:  https://plugable.com/drivers/prolific/

You’ll want to download the driver and unzip it anywhere on your desktop. Once you have, double click on it to start the installation. 

The installation is relatively simple and no questions are asked. When it’s done you should see something like this:

Now go ahead and connect the adapter. You should get a pop-up looking like this:

Finally Windows should report that the device has been installed:

On Device Manager you should be looking at something like this:

Originally authored by: Bernie Thompson, July 4, 2011

Start by going to the Device Manager. Once there do the following: Expand the Ports ‘(COM& LPT)’

You should be looking at something like this:

As you can see Windows has set the port to COM10. However many legacy applications expect the port to be between 1-4. Let’s change that:Right click on the device and click on ‘Properties’.

Click on ‘Port Settings’. Then click on ‘Advanced…’.

Once you’re in ‘Advanced Settings for COM10’ on the bottom you can see the ‘COM Port Number: COM10’. Click on that to change it to the lowest possible number (between 1-4).

Then click ‘OK’ on all open Property Windows. Now the device should look like this: