The PAX devices and the iPad devices in the LinkPOS system listen on a port to communicate with each other over the local WiFi network.

If this port is unavailable the devices can't communicate and you get the Listening Port Unavailable error message:

Often this is caused by one of two things:

There are two LinkPOS apps trying to use the same port

The LinkPOS app on the PAX comes in two flavors - a full service app that can perform all the functions of a Point of Sale and a slimmed down version that can only process credit cards. You can only use one of these apps at a time because they listen on the same port.

You can detect this by touching the square button at the bottom of the PAX screen. This brings up the apps running on the device:

If there are two versions of LinkPOS running then that is the cause of this error.

The resolution is simple.

Touch Clear All to close all of the apps.

Then touch LinkPOS to start the LinkPOS app.

Now you will only have one version of the app running and you won't see the listening port unavailable error.

The listening port is unavailable

If there is only one LinkPOS app running but you still get the unavailable port error then the port is being used by another process.

You can reboot the PAX device and then start the LinkPOS app again and that will usually free up the port and LinkPOS can run again.

If that doesn't solve the problem then an invalid port may be in use.

Touch the three lines in the upper left corner of the app

Touch the Settings tab and then the listening port field.

In this case the port is 4. That is an invalid port number. All ports below 1024 are reserved for the operating system and cannot be used.

The default port LinkPOS uses for the customer facing display is 55124.

Enter 55124 in the port field

Then restart the app by touching the square button at the bottom of the screen and touching Clear All. Then touch LinkPOS to restart the app.

When LinkPOS comes back up the error is gone and the app is running properly again.

Now you need to pair with the iPad to get the two devices connected.

Touch the three lines in the upper left corner and follow the pairing instructions to pair with the iPad register.

Pair the PAX and the iPad