Jenny:
Dies this 24 bytes of data accumulate all during one power on session and then get sent to the serial port before the power goes off?

If your answer is NO it then implies that you are collecting this data over multiple power ups of the device. In this case you will be needing some nonvolatile storage. Nonvolatile storage can be provided through the use of a serial EEPROM. One part number available is 93C46. This intefaces to the processor via 3 or 4 port pins and then a software routine is made to read ans write the EEPROM device in a serial manner. The 93C46 can store 64 separate 16-bit values.

There are examples of interfacing these devices on this site.

Michael Karas