Question
How to convert the binary trace result from the GPIB using the command 'TRAC? TRACE1' ?
What is the Structure of FSE/FSIQ Trace Data on the GPIB?
Answer
The default format after *RST is the ASCII format. That means you will get 500
ASCII values, which are separated by commas, in a single string (approx . 6 Kbyte)
that is terminated by a line feed (0A hex).
To use the floating point format you have to send the command "FORMAT REAL,32"
beforehand.
The FSE will answer the query "TRAC? TRACE1" with a binary string of 500 float
values. Each value is in the IEEE 754 single-precision 4 byte (32 bit) format
that is used by almost all compilers (even with Visual Basic).
Therefore no conversion has to be performed, which saves a lot of processing
time. All values are sent with the low byte first, which is the byte order in the
PC world - for HP workstations please check.
The binary string is formatted according to the <Definite Length Arbitrary Block
Response Data> described in the IEEE 488.2 standard.
This format has to be used together with the preceding length information
if an instrument response is in binary format.
The encoding syntax is the ASCII sign # followed by a non-zero digit. This
digit indicates the number of digits that follow and together specify the
length of the binary information. In the case of FSE the preceding
length information is "#42000...", which means 2000 bytes (500 x 4 bytes) of
binary information.
The C programming example below uses a single-precision FLOAT array to read
the information without any conversion from the IEEE bus. The preceding length
information is simply skipped by a 6-byte offset in the array.
/* DEFINES ****************************************************************/
#define MAX_NOF_POINTS 500 // max. number of points per sweep
#define MAX_NOF_DATA_BYTES (MAX_NOF_POINTS * sizeof(float) + 100)
/* VARIABLES **************************************************************/
UINT8 sweep_data[MAX_NOF_DATA_BYTES]; // stores the data loaded from FSEx
float *sweep_data_ptr= (float *)sweep_data; // pointer to sweep_data
/* Procedures **************************************************************/
int read_sweep_data(void)
/*
SPECIFICATION: read new data from FSEx
Trace data in binary format are 500 float values (2000 bytes)
preceded by 6 bytes
which are used as a length specifier (IEEE488.2 format):
#42000...
PARAMETERS: none
SIDE_EFFECTS: none
RETURN VALUES: 0 if successful
1 if there was any error
****************************************************************************/
{
if (write_to_analyzer("FORMAT REAL32;:TRAC? TRACE1")) // send data of Trace
1 in binary format
{
sprintf(error_msg, "IEEE-Error: TRAC? TRACE1");
return 1; // there was an error
}
if (read_from_analyzer(sweep_data, MAX_NOF_DATA_BYTES))
{
sprintf(error_msg, "IEEE-Error: reading sweep data");
return 1; // there was an error
}
if ( (ibcnt < 2006) || (ibcnt > 2007) ) // number of received bytes
{
sprintf(error_msg, "IEEE-Error: reading sweep data (incorrect byte count)");
return 1; // there was an error
}
sweep_data_ptr= (float *) &sweep_data[6]; // set pointer to data start
return 0;
}