If you are referring to the Recording Rate, the RCX3 has a fixed Recording Rate of 5 Seconds. It is not likely this will be changed via a firmware update.
The RC3 GPS has a 1 second Recording Rate.
The RS800CX and RCX5 offer a selectable Recording Rate from 1, 2, 5, 15, or 60 seconds. Their default setting is 5 seconds.
The RS800CX also offers R-R Data Recording for recording Heart Rate Variability at a resolution of one millisecond!
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