Here are the rules to understand when a record length is available: For the "short" record lengths: 100k record lengths: Time/Div <= 40us (40 us/div and faster) 125k record lengths: Time/Div >= 100us (100 us/div and slower) For the "long" record lengths: 1.00M record lengths: Time/Div <= 400us (400 us/div and faster) 1.25M record lengths: Time/Div >= 1ms (1 ms/div and slower) The reason for the longer than advertised record lengths at slower timebases is so the instrument offers the advertised 1 M record length at all timebases.  The formula to calculate the record length for an acquisition is: Time *Sample Rate = Record Length. The design of the MSO 2000 and DPO2000 series has fixed sample rate possibilities (e.g. 1GS/s, 500 MS/s, 250MS/s, 125MS/s) and keeps time/div numbers consistent (i.e. 1, 2, 4). So the record length has to change since the other two "variables" in the equation are kept constant (Time*Sample Rate = Record Length where Time is the time on screen or time/div * 10).