Why does my Monza X chip backscatter an unexpected EPC?

Do you have a Monza X tag that inventories with an unexpectedly long EPC? In the image below, the EPC length was set to 96 bits but the tag inventoried with a 208-bit EPC. You might wonder how this is possible - this article will explain this scenario, why it is happening and how to change the EPC.

Figure 1. Monza X-2K tag with an EPC length of 208 bits

The tag chip is responding in the way that it is designed to respond to a FastID™ inventory.

FastID mode makes TID-based applications such as authentication practical by boosting TID-based inventory speeds by 2 to 3 times. Readers can inventory both the EPC and the TID without having to perform an access command.

With FastID, the tag chip backscatters the EPC and TID together during an inventory (along with a CRC). Once a tag chip receives a command to respond using FastID, it continues to respond in that method (EPC + TID) until it either 1) loses power or 2) receives a command to stop responding using FastID. If using a Monza X tag in battery assisted passive (BAP) mode with the tag chip powered (VDCI set high), once the chip is set to respond with FastID the situation shown in the figure above may occur. Below is an example of the process to force a Monza X-2K or X-8K tag into this scenario:

1. Set a Monza X tag into BAP mode (ie, connect the battery on a Monza X development kit)
2. Run ItemTest connected to a Speedway Revolution reader and a reader antenna (ie, an Impinj Mini-Guardrail)
3. In ItemTest, set FastID (in Reader Settings -> Configure as shown in the image below)
4. Click "Start" to inventory the tag and set FastID
5. Click "Stop"
6. Change the configuration to not use FastID
7. Click "Start" to inventory again.

Under this condition the tag will backscatter the longer EPC plus the TID seen in figure 1. Note that de-selecting FastID in ItemTest does not change the reader setting to send a command to unset FastID.

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Figure 2. Set FastID in ItemTest

There are two simple ways to make a Monza X BAP tag in this state return to not backscattering a long (FastID) EPC: 1) disconnect DCI power to the Monza X BAP or 2) send a command to disable the FastID response.

If you are unable to disconnect power to the tag chip, the simplest method to disable the FastID command is to send a command to the tag to use the Impinj feature TagFocus without having FastID selected. FastID and TagFocus are set using the same command, so if TagFocus is used and FastID is not, the FastID mode will be turned off. After this step, the tag can be inventoried again in any search mode and will respond with the expected EPC length.

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Figure 3. Setting TagFocus and disabling FastID

Setting the search mode back to single target or dual target with FastID now off will result in a response such as what is shown in the figure below.

Figure 4. Inventory results from a Monza X BAP tag with FastID turned off (default state)

This article has outlined how Monza X tag chips respond in battery assisted passive mode with FastID enabled.