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Theory of Operation
5.4 Mechanisms for FASTBUS Readout
To avoid possible corruption of an event that has not yet been readout, only sixty three consecutive events
may be buffered before the circular buffer becomes FULL. When this condition exists, the MTD Read
Pointer must be advanced, either by issuing an LNE or manipulating CSR16 before additional events can
be buffered. Note when the board is full CIP will lock active until the LNE. Several broadcast operations
are implemented to monitor the 1881M buffering status.
Sparse Data Scan (case 3 or code 09
are buffered and available for readout
Device Available Scan (case 3a, code 19
buffer is Empty.
1881M Unique Sparse Data Scan (case 8c, code CD
has meaning after a Load Next Event command has been executed. It is used to determine if the
event just loaded has unsuppressed data words. Note: all buffered events will have at least a
header word.
5.4.1 Load Next Event
The Load Next Event Load Next Event (LNE)) command is provided to prepare the module for readout
after one or more events has been completely buffered. Assuming there is at least one event in the buffer, a
LNE command advances the Read Page (RP) to the next buffer (the buffer to be readout), clears the Read
Page Address (RPA), and copies the word count found in the header (of the event to be readout) to CSR5.
A LNE issued when no events are available to readout is ignored since this would result in either an old
event being loaded or CSR5 loaded with a meaningless value.
In order to reduce the time required by the module to begin a block transfer, LNE can also optionally prime
the two stage readout pipeline (see figure 5-1). In the module's default data space addressing mode, this
action is transparent to the user. In MTM however, the result of this action in to increment the data space
NTA twice prior the first data transfer. Priming of the readout pipeline does not prevent access to any of
the 1881Ms internal registers.
5.4.2 Block Transfers from Data Space
The normal method of reading event data out of the buffer memory is FASTBUS Block Transfer. In fact,
in the default data space addressing mode of the unit, block transfers (in combination with the Load Next
Event command) are the only viable method to access event data.
In all cases, FASTBUS block reads are controlled by CSR5 and the Read Page Address (RPA). RPA may
or may not be visible to the user as the Data Space NTA depending on the operational mode of the
module. CSR5 is decremented with each block read transfer until CSR5 = 0, resulting in SS = 2.
A typical implementation for a crate full of ADCs might be as follows:
•
Determine if data is available for readout. This determination would be made based on either
Sparse Data Scans or trigger information.
•
Issue a broadcast LNE to prepare all ADCs in the crate for readout. If readout of a particular
event is not required, additional LNEs can be issued to effectively skip an event in the buffer
(providing additional events are available to readout)
•
Address each module in data space and perform a block read.
Re-reads of a particular buffer are possible but not directly supported. In order to re-read a buffer when not
in MTM, the Read Page (RP) must be set to the buffer preceding the buffer to be re-loaded by writing to
CSR16. A LNE must then be issued to reload CSR5 and reset the Read Page Address (RPA).
July 23, 1998
1881M
): Assert Tpin on following read cycle if one or more events
h
): Assert Tpin on following read cycle if the circular
h
): Assert Tpin if CSR5 = 1. This scan only
h
5-6
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