UHF Transponder Memory Structure

To utilize a UHF transponder, it is necessary to understand its memory structure. The structure is defined by the ISO/IEC 18000-63 standard and by the EPCglobal of GS1, so compatible readers conforming to the same standard would be able to access the memory content.

Please note that this document explains the main memory structure of UHF transponders. Depending on the transponder, there may be differences and special features that are not covered here. Further details can be found in the data sheet of the respective transponder.

The following picture shows a schematic structure of an UHF transponder according to ISO/IEC 18000-63 and EPCglobal.

An UHF transponder provides four different memory banks that are described below.

The reserved memory bank contains the kill and access passwords. These passwords can be set manually and can be locked to prevent unauthorized changes. For more details on the kill and locking process, see this Knowledge Article.

The kill password is 4 bytes long and is used to permanently destroy transponders. The default kill password is 0x00000000. To use the kill command, you need to set a password other than the default.

The access password has the same size of 4 bytes. It is used to restrict the access to the transponder data. The default access password is 0x00000000 as well. 

The UII (Unique Item Identifier) memory bank consists of various parts. These are explained in the following.

The CRC is a checksum that is calculated from the content of the PC word and UII. This is used to check if the transfered data is correct.

The PC word contains information of the format of the UII/EPC. The first five MSBits (Most Significant Bits) indicate the Length of the UII/EPC.

• EPC Length
  Indicates the length of UII/EPC in words
• UMI (User Memory Indicator)
  Indicates whether a User Memory is active
• XI (XPC_W1 Indicator)
  Indicates whether a XPC (Extended Protocol Control)  is active
• NSI (Numbering System Identifier)
  
  • If the transponder memory follows the EPCglobal structure (T=0)
    Bit 18h to 1Fh is used for AFI (Application Family Identifier) as defined in EPCglobal Data Standards
  • If the transponder memory follows the ISO structure (T=1)
    Bit 18h to 1Fh is Reserved for Future Use (RFU) as defined in ISO/IEC 15961

List of common used PC words and corresponding EPC Length

UII (Unique Item Identifier):

The Unique Item Identifier (UII) is a term used when the transponder memory adheres to the ISO structure. In this context, the UII represents the specific identifier data stored in the RFID tag, besides the CRC (Cyclic Redundancy Check) and PC (Protocol Control) word. The UII is often used to give a transponder a unique name so it can be identified distinctly, which is crucial for inventory management, asset tracking, and other identification applications.

EPC (Electronic Product Code):

The Electronic Product Code (EPC) is used when the transponder memory follows the EPCglobal Data Standard, which is a widely adopted framework in the RFID industry. The EPC serves a similar purpose to the UII, providing a unique identifier for each item. It is stored in the RFID tag's memory along with the CRC and PC word. The EPC is designed to be a globally unique identifier, enabling seamless tracking and identification of products across different systems and organizations.

EPCglobal is a global standard that defines standardised formats and protocols, so transponders from different manufacturers and systems can be understood. Depending on the used coding scheme RFID reader can generate the Unique Ressource Identifier (URI) This enables seamless communication and interoperability in RFID applications. See the EPCglobal Data Standards for further information.

Key Differences:

• Terminology: The term UII is used in the context of ISO-compliant tags, whereas EPC is used for tags that follow the EPCglobal standard.
• Purpose: Both UII and EPC serve the same fundamental purpose of uniquely identifying items, but they do so within the frameworks of their respective standards.

XPC (Extended Protocol Control) words are optional. They provide additional protocol control information that extends the capabilities of the basic PC word in UHF RFID tags. The XPC words are used to manage specific features and functionalities of the RFID tag. See EPCglobal Data Standards or ISO/IEC 18000-63 for further information

The TID (Tag Identifier) memory bank is a special part of an RFID tag’s memory that holds a unique code given by the manufacturer. This code is permanent and cannot be changed.

The first Byte (MSB) of the TID contains the UHF Header E2h according to ISO 18000-6. The following three byte are indicating the tag manufacturer and the tag model. The remaining data is a worldwide unique serial number of the transponder.

The User memory bank is used to store additional information chosen by the user. This can include details like product information, process data, or any custom data. It is flexible and writable, allowing users to update the information as needed. The size of the User Memory depends on the used transponder.

The UHF transponder data can be accessed via the "Transponder Access" page in SOPAS ET or via commands. The following picture shows an example of an accessed transponder in SOPAS ET with a RFU6xx device.