note |
This function is only available on Cobalt series, S5180B, and S5243 models. This function should not be confused with the Memory FIFO function, which stores multiple trace positions on the screen at different points in time. |
Purpose
The FIFO buffer function is a software function available through SCPI automation commands. The function uses a FIFO-type buffer in the PC's RAM to record a series of many identical sweeps triggered by an external trigger.
Capabilities
•The maximum buffer size is about 1.5 GB.
•The unit of reading/writing data in the FIFO buffer is one full sweep.
•The capacity of the FIFO buffer is set by specifying the required number of sweeps.
•The function is compatible with an external trigger having a stable period exceeding the width of one sweep.
•The sweep data is read from the buffer by an external automation program using SCPI commands.
•The scan data can be read after the scans have accumulated in the buffer or immediately after the start of recording, which makes the read cycle almost endless.
•The data includes measurements of all channel traces.
•The data of each trace is written to the FIFO buffer in real and imaginary part format, regardless of the format set for the trace.
• The data of each trace includes all processing stages up to the formatting stage (See Internal Data Processing and Internal Data Arrays).
•When a buffer overflow occurs, the overflow flag is set, and the sweep is ignored. The buffer overflow occurs when the difference between the number of written and read sweeps exceeds the buffer capacity.
•The function has a built-in miss detector of the external trigger.
limitations
•Display refresh is disabled when the function is running.
•The number of measurement points must be less than 3000.
•Only one channel should be open.
•Program (BUS) and manual trigger are not available. The internal trigger is supported, but is of little use in this function.
•Averaging trigger is not supported.
•SCPI commands that change analyzer settings cause the function to automatically close and stop writing to the buffer, similar to the SYST:FIFO OFF command.
Indication
The analyzer status bar displays a “FIFO n%” indicator when using the FIFO buffer function. Where “n%” is the percentage of the FIFO buffer full. The indicator turns red when the FIFO buffer is full.
Data Format
The read unit of the FIFO buffer is a floating point array that contains data from all channel traces for all measurement points of a single sweep.
The array size is:
Size = 2*N*M,
where:
N -– number of measurement points
M -– number of traces
Array Format
Point 1 |
Point 2 |
... |
Point N |
||||||||||||
Trace 1 |
... |
Trace M |
Trace 1 |
... |
Trace M |
Trace 1 |
... |
Trace M |
|||||||
Re |
Im |
Re |
Im |
Re |
Im |
Re |
Im |
Re |
Im |
Re |
Im |
||||
FIFO buffer size in bytes
The FIFO buffer size in bytes is:
Size = S*N*M*16,
where:
S – maximum number of sweeps in the FIFO buffer set by SYST:FIFO:SWE:CAP command.
N – number of measurement points.
M – number of traces.
16 (bytes) – the total size of the real and imaginary parts of the complex number in the FIFO buffer.
Features of trigger initiation mode
The FIFO buffer function is only compatible with the "Continuous" trigger initiation mode. When the SYST:FIFO ON command starts recording to the FIFO buffer, the “Continuous” trigger initiation mode is automatically turned on. In this case, the current settings of the initiation mode don't matter. The analyzer returns to the previously set trigger initiation mode after finishing writing to the FIFO buffer.
note |
Recommendation for user programs that use a FIFO buffer. Use the INIT:CONT OFF command to preset the trigger initiation mode to “Stop”. This will reduce the time it takes for the Analyzer to prepare for the first trigger after receiving the command to start writing to the FIFO. |
Detector Miss of External Trigger
The FIFO buffer function is designed in such a way as to reduce the possibility of missing an external trigger. Possible misses are monitored by the detector miss of the external trigger function, which is turned on if necessary.
When a missed trigger is detected, this function performs the following actions:
•increases the counter of missed triggers
•inserts a dummy sweep into the FIFO buffer, the data into which is copied from the previous sweep
•saves a list of the numbers of inserted dummy sweeps in a separate array
The number of skipped triggers and a list of their numbers can be read using SCPI commands. Information about missed triggers remains available after the FIFO buffer is turned off with the SYST:FIFO OFF command. Information about missed triggers is cleared when the FIFO buffer is turned on with the SYST:FIFO ON command, as well as when the buffer is cleared with the SYST:FIFO:DATA:CLE command.
Limitation: The function cannot detect the miss of the last trigger in a sequence if there is no measurement after that trigger. To overcome the limitation, two methods can be used:
•Use the number of triggers, one more than required.
•Compare the number of sweeps read with the number of triggers submitted. The number of sweeps read will be one less than the number of triggers submitted if the last trigger is missed.
Analyzer “Ready for Trigger” Output Signal
The “Ready for Trigger” output signal can be enabled and used for the FIFO buffer function, similar to the analyzer's normal mode. If this signal is used, then the external trigger cannot be missed. Instead, the external trigger source will delay the trigger until it receives a ready signal from the analyzer.
Program examples
Examples in C are given in the application folder ..\Programming Examples\SCPI\Buffer FIFO.
