High rate acquisition
This document discusses the acquisition of multiplexed data at rates up to ~20 kHz. This is different from 50 MHz Raw-mode readout .
Timing parameters
The frequency of internal MCE frames, i.e. the multiplexing rate, is determined by the parameters "num_rows" and "row_len". The multiplexing rate is
f_sam = 50 MHz / num_rows / row_len
The parameters "num_rows" and "row_len" are present on all MCE hardware cards. When writing these parameters, write to the "sys" card, e.g. "wb sys num_rows 8". When reading the value it is ok to just use the clock card value, i.e. "rb cc num_rows".
The parameter "data_rate" determines how often an internal data frame is sent to the readout PC and written to disk. This parameter is an integer indicating how many internal frames to process before outputting a frame to the PC. The output frame rate is thus
f_out = f_sam / data_rate
Readout parameters
Regardless of how many rows are being multiplexed, it is possible to configure a different set of rows to be output. The advantage of this is that by sending less data to the readout PC, the readout frame rate can be increased.
The parameters that determine the form of the output data are "num_rows_reported" and "readout_row_index". The "num_rows_reported" parameter, which lives on the clock card, determines the number of rows of data which should be returned to the readout PC. The readout_row_index parameter, which lives on each readout card, determines the row offset into the frame data at which readout should begin.
For example, to return frames containing only data from row 5, one could set "wb cc num_rows_reported 1" and "wb rca readout_row_index 5". In normal configuration, num_rows_reported should be equal to num_rows and readout_row_index should be equal to 0.
Through-put limitations
Bandwidth limitations are most easily expressed in terms of MB/s. The current limits on data throughput from the MCE are in the neighbourhood of 4 MB/s.
| num_rows | row_len | columns | num_rows_reported | frame_size (bytes) | f_sam (kHz) | Maximum f_out (kHz) |
|---|---|---|---|---|---|---|
| 33 | 100 | 32 | 33 | 4400 | 15.15 | 1.4 |
| ... |
