This command should be used to check for the presence of a card. It is special in that it can be used to reset the card. Most other commands will return an error when a card change has been detected and the use of this function is therefore required.
Background: We want to keep the client clear of handling card changes between operations; i.e. the client can assume that all operations are done on the same card unless he call this function.
Return the serial number of the card using a status response like:
S SERIALNO D27600000000000000000000 0
The trailing 0 should be ignored for now, it is reserved for a future
extension. The serial number is the hex encoded value identified by
0x5A tag in the GDO file (FIX=0x2F02).