The L2 Framework
                        ----------------

                                                Created: 03-Feb-2005


The L2 Framework is the interface between the L2 Trigger Decisions
and the DZero Data Acquisition System. [footnote L2FW drawing]

The L2 Framework keeps a queue of all L1 Accept Decisions
awaiting their L2 Decision, namely L2 Confirmation or L2 Rejection.

For each event accepted by the L1 Framework the L2 Global Processor
sends a bit mask to the L2 Framework to individually confirm or reject
each of the 128 L1 Specific Triggers. This mask is available
at the L2 Framework after some latency for data collection,
transfer, and processing time, but always in strict chronological order.
The L2 Framework is not allowed to change the trigger decisions made
by L2 Global Processor.

The L2 Framework will use and translate the L2 Global bit mask
to determine which of the Geographic Sections having already received
a L1 Accept Decision now needs to receive a corresponding
L2 Accept or L2 Reject.  These decisions are sent to the Geographic
Sections over the Serial Command Link [footnote SCL].
Each L2 Processor crate is also a Geographic Section and thus receives
the final L2 Decision over its own SCL connection.

The L2 Framework also listens to the L2 Busy status from each
of the 128 Geographic Sections.  Asserting L2 Busy means that the
Geographic Section has run out of storage space to record the data
for another L2 Accept. When the L2 Busy is asserted for some Geographic
Sections about to receive a L2 Accept, the L2 Framework stalls sending
this and subsequent L2 Decisions untill all the L2 Busy signals involved
become deasserted again.

footnote L2FW = http://www.pa.msu.edu/hep/d0/ftp/l1/framework/drawings/the_layout_l2_fw_cards.ps
footnote SCL = http://www.pa.msu.edu/hep/d0/ftp/scl/scl_description.ps