Windmill Software Ltd
Windows Engineering Software

USB-Serial Converters

Home
Catalogue

Tips on Using USB-Serial Converters for Data Acquisition

Many laboratory instruments can send data to a computer over RS232 links. Driver software on the computer collects the data and makes it available to applications like Windmill Logger or Microsoft Excel. Windmill offers two serial drivers: COMIML (their latest driver) and LabIML (quite old now but free).

As USB ports are replacing RS232 ports on computers, people are increasingly using a USB-Serial converter to access data from their instruments.

The converters are simple to use but sometimes need careful setting up if they are to work properly. Here are some tips on using a USB-Serial converter.

1. Plug the converter into the PC's USB port and install its driver software. If you don't have any driver software then check with the manufacturer.
2. Windows will assign the USB converter a COM port. Check this by opening Windows Control Panel and choosing first System then Device Manager then Ports. It will look something like "USB serial port(Com X)", where X is the COM port number assigned. (A question mark indicates that there is a problem with the drivers or the connection.)
3. You may find that Windows assigns your converter a COM port number that is higher than your software can handle (This is especially relevent if you are using the free Windmill RS232 driver, LabIML.). If so, use Device Manager to reassign the COM port to a lower number. Select your converter and click Properties. Choose Port Settings then Advanced. Make a note of this number and the other settings listed: you will need to enter them in your data acquisition software.
4. Windows may list all the lower COM port numbers as in use. This is not always so: look at the computer's actual COM ports physically available and you may find a lower port number.
5. If your USB-Serial device is "designed for Windows" then it will have a unique serial number. Windows reads this and uses it to remember which driver and what COM port number to use for that device. This means that when you unplug the converter, Windows continues to consider its COM port number in use. You can over-ride this with Device Manager. It also means that if you swap converters the port number assigned will be different.
6. From the computer's perspective, the adapter is an active USB device. What this means is that it will only be available, or listed as an option, if it's plugged in.
7. When using a USB-Serial converter, you need to make sure that it is providing data in the way expected by your data acquisition software. The serial settings must be the same.
   1. To find out the communication settings use Control Panel's Device Manager as detailed in point 3 above. Enter these in the COM Port Settings box (COMIML) or Comms Settings (LabIML).
   2. If you have set your DAQ software to continually acquire data you must make sure that your USB-to-Serial converter is continually providing data, and not buffering it. How you do this depends on your converter. For an Edgeport converter, for example, you need to use its utility program to set its ports for "low latency" and "fast read and writes".