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Serial Number in Tacx Utility App Does Not Match Serial Number on Neo

When looking at the details of your Neo in Tacx Utility App Testing Dashboard, you may notice that the line which reads “Serial Number Device” does not match the printed label affixed under the non drive side leg.

As strange at this may seem, this is normal under certain circumstances.

These are images from the same Neo 2T, but the serial number on the physical label doesn’t match the serial number that is displayed in Tacx Utility App.
Photo Credit: Gregor Taschwer

Root Cause

The ANT+ channel ID for any given device comprises four bytes: 1 for the transmission type, 1 for the device type, and 2 for the device number. The two-byte device number is what we’re interested in, because that’s what we typically see as the ANT+ ID when we’re pairing our devices. Because this value is two bytes, it has a range of 65,536 (2^16).

If it’s already at its maximum value, what happens if you add 1 to it? Well, in decimal numerals, like the Neo serial number label, you get 65537, but in computer systems — you get the decimal value modulo the range.

65537 modulo 65536 = 1

So if you have a Neo with serial number 65537, your ANT+ ID (and what’s shown in the Tacx Utility App) will be 1.

Going back to the example in the photos, the serial number on the label is 65897.

65897 modulo 65536 = 361 — exactly what’s shown in the Tacx Utility App.

This overflow behavior will always be the case, even if the Tacx sells 500 thousand or a trillion. The number shown in Tacx utility app (and other software that communicates via ANT+) will be the serial number on the label modulo 65536, and that number will always be between 1 and 65536.

So the serial number does actually match, but you see it represented as an artifact of integer overflow.

Additional Information

As a result of this behavior, each time the serial number exceeds the range of the 2-byte ANT+ device number, the ANT+ ID of that particular Neo unit will be a duplicate that conflicts with another Neo unit somewhere in the world. It is extraordinarily unlikely that any two such devices will ever be in the same place, but it’s hypothetically possible.

ANT+ also has a feature called “extended device numbers” that allows manufacturers to extend the range of the ANT+ device number from 16 bits to 20 bits. While that does not eliminate the hypothetical problem of duplicate ANT+ IDs, the range is extended from 65536 to 1048575. It is presumed that Tacx considered this option, but deemed it unnecessary on the infinitesimally small probability that any two such Neo devices would ever be in the same place.

Special thanks to Gregor Taschwer for discovering this feature of his Tacx Neo 2T, and Martin Smits for letting us know that it is, indeed, normal behavior.

Updated on December 7, 2019

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