NXT Functions Datalog

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The NXT has a data logging capability similar to that found in the RCX. This capability allows you to save variable and sensor values in a log during program execution. This is useful for collecting data during measurements. It is also useful for program debugging where you may want to analyze the value of a particular variable after a program has been run. There’s room for storage of about 5,000 data points.

The data points are stored into RAM. They can be subsequently copied to a NXT flash file. They are not immediately stored into a flash file during program execution because writing to a file in the flash memory can take 3 to 6 milliseconds which could disturb performance of the user application.

There are utilities within the ROBOTC IDE to upload the datalog and display in a PC window. These can be found under the “debug display” and the “NXT -> Files Management” display.

Color Key


void AddToDatalog(const long data)
(void) Adds an entry to the datalog.
Parameter Explanation Data Type
data An integer or long variable to print to datalog long
int i;               // create variable 'i'
for(i=0; i<10; i++)  // cycle through "for loop" ten times (0-9), incrementing 'i' with each iteration:
  AddToDatalog(i);     // add variable 'i' to the datalog with each iteration of the loop


word bHideDataFiles
(word) Boolean flag to indicate whether data files should be hidden or shown in the NXT GUI. Note that this is defined as a word but is treated as a bool.
bHideDataFiles = true;   // data files will be HIDDEN and NOT SHOW on NXT GUI   
bHideDataFiles = false;  // data files will be SHOWN on NXT GUI


word nDatalogSize
(word) The current size allocated to the datalog.
const int size = 2000;  // a constant int variable 'size' of size 2000    
nDatalogSize = size;    // create a new Datalog of size, 'size'


word nMaxDataFiles
(word) Maximum number of saved datalog files (DATAnnnn.RDT) allowed on a NXT.
nMaxDataFiles = 15;                                     // allow up to 15 Datalog files on one NXT   
nxtDisplayCenteredTextLine(3, "Max Datalog Files:");    // display on a centered textline on line 3 
nxtDisplayCenteredBigTextLine(4, "%d", nMaxDataFiles);  // (will display, "15")


word nMaxDataFileSize
(word) Maximum size (in 100 byte units) of all datalog files saved on the NXT.
nMaxDataFileSize = 10;                                     // 10*100 = 1000 = roughly 1 kilobyte   
nxtDisplayCenteredTextLine(3, "Max Datalog Files:");       // display on a centered textline on line 3 
nxtDisplayCenteredBigTextLine(4, "%d", nMaxDataFileSize);  // (will display, "1000")


const word nUsedDatalogBytes
(word) The number of bytes currently containing data in the datalog.
nMaxDataFileSize = 10;                           // 10*100 = 1000 = roughly 1 kilobyte  
while(nMaxDataFileSize - nUsedDatalogBytes > 0)  // while there are still bytes left (0 < bytes < size):
  AddToDatalog(data);                              // write to datalog (fill it up!)


void SaveNxtDatalog()
(void) Saves the existing datalog from RAM memory into a NXT file named "DATAnnnn.rdt" where 'nnnn' is numeric and increases by one on every save.
SaveNxtDatalog();  // saves the Datalog