The Discretization step size is a key parameter that is used to transform all continuous states to the discrete state-space domain and to configure the nominal execution rate of the digital control loop.
The Target buffer size setting, described below, determines how much memory is allocated to store signals for the external mode.
The Number of samples in the External Mode tab then specifies how many of those signals are displayed. If the specified Number of samples exceeds the samples available in the allocated memory then all samples are displayed. For the TI C2000 TSP one would typically set this at the Nsamples calculated in the target buffer size (or above that value for the sake of convenience). For the RT Box target the buffer size is much larger and so one would be more judicious in how this value is set.
If you want to capture data over a longer time, but cannot capture more samples due to memory limitations, you can increase the Decimation setting in the External mode tab. The decimation says that every Nth sample will be stored in the external mode buffer.
With that in mind, for a given number of words (Nwords), number of signals (Nsignals), discretization step size (Tdisc), and decimation setting (decimation) you can perform the following calculations, provided as an Octave/Matlab script:
Ts = 1e-4; % 10 kHz example
Nw = 2000; % Target buffer size
Nsignals = 2; % Number of scope traces & displays
Decimation = 1; % Every Nth sample
Nsamples=(Nw/(Nsignals*2)-1); % Number of samples for each scope trace
Tscope=(Nsamples-1)*Ts*Decimation; % Total time duration of scope
>> Nsamples = 249
>> Tscope = 0.024800