The short answer: Download the latest version of Logger Pro or Data Logger from the our website at:

Another consideration is what version of the operating system you are using. If you are using OS 8.5 or older, we suggest that you upgrade. Apple has made several improvements in the way their operating system handles serial communications. If you have not upgraded to 8.6 already, it is a worthwhile upgrade. OS 8.6 can be downloaded free from Apple at:

If you are using OS 9, make sure you have OS 9.0.4. If your Mac is connected to the internet, you can get the update for free by opening the “Software Update” Control Panel.

The long answer: Several Macs that came out in the 1997-98 time frame had a series of problems with the ASIC that controls the serial port. Apple made a few changes in both the hardware and the software to fix the problem. The final software fix for all of these computers is integrated into OS 8.5 and newer.

The same fix can be added to earlier versions of the OS by adding the Serial DMA extension to the extensions folder, restarting, and rebuilding the desktop. To rebuild the desktop, hold down the Apple and Option keys (the 2 to the immediate left of the spacebar) for the entire time the computer is starting up. A dialog box will come up asking you if you would like to rebuild the desktop. Click OK, let it run it’s routine, and everything should be set.

If you don’t have the serial DMA extension, you can download it for free from the Vernier web site at:

We include this extension with both Data Logger 4.5.9x and Logger Pro versions 1.1 and newer.