1. TTC signals: Global Trigger and Global Muon Trigger
Manfred Jeitler
TTC Meeting October 2006

2. the Global Trigger as source of TTC signals
Global Trigger has to receive clock and orbit signal
from TTCci over fiber, or
from TTCmi over Lemo cables
Trigger Control System (TCS) produces L1A and BGO signals
at start of run
orbit counter reset, event counter reset, resynch, start
at end of run
stop
for resynch during run, hard reset etc
“private” signals for each DAQ partition
all BGO signals are created at specific bunch crossing number
to be selected in TCS, can be different for each DAQ partition
L1AOUT sends L1A and BGOs to all user TTCci’s
send over the (in)famous “Hirose” cables
8 wire pairs for the 4 BGO bits, L1A, BCRES, clock, strobe

4. the Global Trigger and Global Muon Trigger as users of TTC signals
the TCS board could distribute triggers and BGOs to all the boards in the GT crate (including GMT) over the backplane
we need these signals for the readout, like everybody else
but we prefer to receive all signals via the fiber from our TTCci
this allows to check right away if the signal distribution works
GT uses TTCci in a “transparent” mode
we want to receive BGO signals in exactly the same way as we send them (same bit patterns)
but everybody is free to program “their” TTCci the way they need/like it

5. L1A, BGO and clock distribution
GT crate
L1AOUT
TCS
GMT
TIM
TTCmi
clock + orbit
L1A
+ BGO
L1A, BGOs; clock?
TTCci
TTCex

6. the question of the clock distribution
the idea of the TTC group:
get L1As, BGOs and clock from L1AOUT into TTCci
“so there should be no danger of clock shifting with respect to L1A and BGO signals”
TTCci sends this to TTCex, which gets directly the “best possible” clock from TTCmi (or its successor)
detectors need by all means this direct TTCmi clock because the TCS system could introduce a slow drift of up to 2 ns
but:
we really do not expect the clock to shift into the wrong bunch crossing
if this should ever happen, the Global Trigger would notice it right away in its monitoring
but the clock from the TCS may also have a jitter
(on top of a drift)
the TCS was never meant by its designers to distribute a clock !
so why use this clock at all?
please use TTCmi clock right away in TTCci !