USB to Serial on Yosemite

2-minute read
Tech
Warning
This is an older post that I imported from my previous blog. I have not validated whether the procedure below is still relevant for Yosemite, but it most certainly will not be for newer macOS releases!

There have been numerous blog posts written on getting USB to Serial adapters working under Mac OS X — and specifically Yosemite. However, none of them gave me the absolute answer on how to get it working perfectly. And when a colleague of mine asked me to help him get his working on his Mac a couple of weeks ago, it seemed all was lost for him as well.

We followed the instructions on a couple of different web pages without any success and finally we just copied the driver I had running to his Mac and everything was working all of a sudden! Magic.

The driver I’m using took me a while to get right as well when I first started using a Mac some years ago. But since then it hadn’t failed me. Until of course Yosemite came along and required drivers to be signed:

com.apple.kextd[19]: ERROR: invalid signature for com.prolific.driver.PL2303, will not load

Luckily this was easily solved by running the following command and rebooting:

sudo nvram boot-args="kext-dev-mode=1"

The Correct Driver

Basically, there are two drivers available on the Internet for the Prolific USB to Serial chipset. The first is the Open Source version available on GitHub and the second is from a website called Plugable. As far as I can see the results are mixed for any of the adapters, but I’m using the latter.

When I first downloaded and installed this driver, it didn’t work until I made some changes in the Info.plist file included in the driver. To do so, I first unloaded the driver and opened the Info file:

sudo kextunload /System/Library/Extensions/ProlificUsbSerial.kext
sudo vi /System/Library/Extensions/ProlificUsbSerial.kext/Contents/Info.plist

In it you will find the 067B_2304 key which includes a certain idProduct and idVendor. These settings are the ones I had to change to reflect the following values:

    <key>067B_2304</key>                                                       
    <dict>                                                                     
        <key>CFBundleIdentifier</key>                                          
        <string>com.prolific.driver.PL2303</string>                            
        <key>IOClass</key>                                                     
        <string>com_prolific_driver_PL2303</string>                            
        <key>IOProviderClass</key>                                             
        <string>IOUSBInterface</string>                                        
        <key>bConfigurationValue</key>                                         
        <integer>1</integer>                                                   
        <key>bInterfaceNumber</key>                                            
        <integer>0</integer>                                                   
        <key>idProduct</key>                                                   
        <integer>8296</integer>                                                
        <key>idVendor</key>                                                    
        <integer>24969</integer>                                               
    </dict>

And when finished you can reload the kext-file: sudo kextload /System/Library/Extensions/ProlificUsbSerial.kext

Tested Devices

The known-working USB to Serial adapters for this driver are the following:

  • Sitecom USB to Serial Cable 0,6m CN-104
  • Trendnet USB to Serial Converter TU-S9 (Version v1.xR)

Please let me know on Twitter if it works for your device as well, or if you have any other questions.