RFID Encoding and available memory
Among the services we offer is the RFID Encoding. Let's see what you can program in an RFID Tag, and how much memory you need if you use Hex or ASCII encoding.
Before getting to the heart, please note that, of the 4 types of RFID memory, only 2 are programmable: the EPC memory and the user memory (if present). The reserved memory and the TID, in fact, are not rewritable.
RFID UHF Tags are provided with the EPC memory already encoded by manufacturer with a string of code. However, this does not mean that the memory of the Tag could not be rewritten. The user memory, on the contrary, is usually empty.
Why you might need to encode the EPC memory
Depending on the type of chip (and in particular the manufacturer), the EPC can have different factory settings. Some manufacturers supply chips all programmed with the same code string; others, with a unique code. If you want to use the factory set EPC, check how it is delivered.
The EPC can be used for different purposes: to uniquely identify an asset, or to contain specific information, such as a counter, or a product code. In the second case, it is evident that it must be overwritten; while in the first case, if each EPC is already programmed with a unique code from the factory, it is possible to skip a step and use the already programmed string. For the same purpose (the association of a code to a certain asset), it is also possible to use the TID of the chip.
These aspects are to be assessed on the basis of one's needs, the size of the various memories, and also the foreseen phases of applications of the Tag. In fact, if you can save the programming step, you must also consider that, in order to associate it, the Tag must still be read: you need to understand which solution requires less time, and therefore less waste of resources.
Tags that are actually encoded with a unique and random EPC can potentially be used without being reprogrammed, because the chances of finding two identical codes are zero. For example, the EPC number on an Alien chip is created using a combination of the ID of the wafer the chip is on, the position of the wafer, and a portion of the last 35 bits of the TID number. Together, these elements create a unique 32-bit serialization factor at the end of the EPC.
Unencoded tags with a unique and random EPC must be re-encoded before use. Some Tags are sold, as we have said, with the same EPC for the entire Tag reel; others have randomized EPCs but are not guaranteed to be unique.
So first you need to determine if the selected tags have a unique, random or serialized EPC. This can be done by checking the technical specifications of the RFID Tag.
Bit, Hex and ASCII - How much memory is needed
The memory of RFID chips is usually expressed in bits. Bits are basic information units. A bit can only take the values zero and one. The bits are transmitted in groups of 4 digits and this binary system is the basis of all digital messages.
Starting from the bits, in fact, more complex messages can be created, using two most common encoding formats for RFID tags: Hex and ASCII.
Hexadecimal encoding (also called Hex, or base 16), is a method that uses 16 values: the numbers 0 to 9 and the letters A to F.
The 4-bit groups can assume 16 different values (from 0000 to 1111), and each 4-bit string coincides with 1 hexadecimal character (also called a byte).
When a 96-bit EPC is read, 96 values of 0 and 1 are transmitted to the reader. These correspond to a quarter of the values expressed in Hex. Therefore:
- 32 bits correspond to 8 Hex characters
- 64 bits correspond to 16 Hex characters
- 96 bits correspond to 24 Hex characters
- 128 bits correspond to 32 Hex characters
- 256 bits correspond to 64 Hex characters
ASCII (American Standard Code for Information Interchange) encoding is more complex, meaning it uses 128 values. Each ASCII character, however, requires 2 bytes, i.e. 2 Hex characters, expressed in 8 bits. With ASCII encoding, the values that the string can take include all letters of the English alphabet (upper and lower case), numbers and some special characters, such as the question mark and asterisk.
In summary, each ASCII character requires 8 bits, therefore:
- 32 bits correspond to 4 ASCII characters
- 64 bits correspond to 8 ASCII characters
- 96 bits correspond to 12 ASCII characters
- 128 bits correspond to 16 ASCII characters
- 256 bits correspond to 32 ASCII characters
The memory of the RFID tags is rather limited. Many chips have either 96- or 128-bit EPCs. It is evident that ASCII encoding, requiring more memory, is rarely adopted. Please note that the EPC of a Tag is always encoded in hexadecimal format. So, if you want to use ASCII characters, you need to implement an ASCII-Hex conversion formula in your software for encoding and reading RFID tags.
In conclusion
To conclude, here are our tips.
- Consider whether TID can be used or EPC memory must be used
- Consider whether EPC memory needs to be programmed
- Evaluate the type of encoding required and calculate the amount of memory required
RFID tags with the physical characteristics that meet our needs are not always available with the required chip, so sometimes it is necessary to change strategy. For example, consider using Hex instead of ASCII encoding.
It should also be mentioned that the more expensive Tags often host chips with larger memory than the average. For example, the Confidex Ironside Flag is manufactured with the Monza 4E, which has a 496-bit EPC and 128-bit user memory, so it can satisfy most memory-related demands.