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.