Read-only tags are programmed at the factory with a serial number that can not be changed. This can be useful when you have a high-volume application that involves tagged items moving outside of a company’s control. For instance, if you want to tag millions or billions of pharmaceutical drug bottles, you don’t want the serial number to be changed. If the chips are even a fraction of a penny more expensive than those that can be written to only a single time, that can still add up when you are tagging billions of items.
Tags that can be written once and read multiple times are called field-programmable. That is, you can write your own serial number at the time you use the tag, rather than having the factory write the number to the tag. The benefit of this option is that you don’t need to provide serial numbers to the semiconductor fabricator in advance, so you have greater flexibility. If you decide to change the numbering scheme, or if different factories you own use different prefixes, you can program the tags with no problem. This is not possible with read-only tags encoded at the factory. The cost of this flexibility is a chip that is slightly more expensive. In low volumes, the cost is insignificant. In high volumes, it can be significant.
Tags that can be written multiple times are slightly more expensive. In most cases, companies write a serial number and then lock that number. Some tags have blocks of user memory that can be written to multiple times. This can be used in many ways. Let’s say you are shipping a case of goods from a factory to a distribution center. You might write to the user memory a code indicating to which store the shipment should be sent. That way, when the tag is read, workers immediately know where to direct the product. But a manager might decide to change the order and send it to a different store, and he might overwrite the store code with a new code.
User memory can usually be locked. But locking and unlocking memory involves storing serial numbers for each tag. Managing serial numbers adds complexity to the deployment, so companies must figure out which type of tag best suits their needs from a cost, flexibility and privacy perspective. That’s why tag manufacturers offer a variety of options.
—Mark Roberti, Editor, RFID Journal
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