The T5557 protocol by Atmel supports read and write operations on RFID tags. Here a shorter summary of the T5557 protocol. By that it’s easy to clone tags, like in this example. But at the same time this rises security issues, which is probably the reason why it is not so commonly used (yet). The difference to the successor, the T5567 protocol is simply some improvements in terms of power on reset if the tag enters the field very slowly.