# Trigger Programming for Hall A Spectrometers

```
Notes on Trigger Programming for e94107 and Beyond
===================================================

For Year 2003 and earlier see trigsetup_2003.html.

updated Jan 2004
experts:  R. Michaels, R. Feuerbach, and  B. Reitz

This file describes some details about the Hall A trigger system.
A less detailed general introduction is
hallaweb.jlab.org/equipment/daq/trigger.html

We have circuit diagrams of the single arm trigger
and coincidence trigger (postscript format).
The trigger was refurbished in 2003, it was torn apart in March
to clean all modules, and then in May the S2m scintillator was added
by R. Feuerbach to achieve better time resolution.
Here are same diagrams in PDF format:
single arm (PDF)
coincidence (PDF)

The main triggers (T1 on R-arm and T3 on L-arm)
are formed from an overlap of S1 and S2 scintillators.
This is described in detail below.

The triggers during e94107 (hypernuclear) are

T1 = Main R-arm trigger
T2 = Loose R-arm trigger, for measuring efficiency
T3 = Main L-arm trigger
T4 = Loose L-arm trigger, for measuring efficiency
T5 = coincidence of T1 and T3.
T6 = T3.and.A2.and.NOT.A1 (Clean Kaon Trigger) -- see below
T7 = T6.and.T1
T8 = 1024 Hz pulser

These may be prescaled by editing ~a-onl/prescale/prescale.dat.
The maximum prescale factors are 24 bits for T1-T4 (i.e.
2^24 - 1 = 16,777,215) and 16 bits for T5-18 (2^16 - 1 = 65535).
Meanwhile T9-T12 cannot be prescaled.
The prescale factor is a modulus of these limits, so if you
put ps8=100000, then 100000 % 65536 = 34464 is the prescale factor
you ACTUALLY used.  Be careful.

Let us first describe the MAIN TRIGGER, T1 and T3, which
is based on scintillators.  First the Left and Right PMTs
are AND'd.  There are then 6 (16) signals from each scintillator
planes S1 (S2).  These results are OR'd to make one signal
each from S1 and S2.  Finally, the trigger is the AND of these
two signals.  Here is an ASCII diagram:

S1 - paddle 1 - Left   --\
S1 - paddle 1 - Right  ---AND -->
S1 - paddle 2 - Left   --\       \
S1 - paddle 2 - Right  ---AND --> \ OR of S1
\
\ AND -------> T1 (T3)
/
/
S2 - paddle 1 - Left   --\        / OR of S2
S2 - paddle 1 - Right  ---AND -->

Coincidence trigger T5 is defined as the overlap between
T1 and T3.  Typically the two signals are 100 and 50 nsec
wide respectively, see the halog for the details of the
timing.

For e94107 there are two new triggers: T6 and T7 defined
as follows.
T6 = T3 .AND. A2 .AND. .NOT.A1
where A1(A2) = the summed PMTs from A1(A2) aerogel, attenuated
and put through a discriminator with typically 30 mV level.
T3 and A2 are inputs to a Phillips 764 logical coincidence
unit. The ".NOT.A1" means the signal is used as a veto in a
the same Phillips 764 module.

The timing at the trigger supervisor will be such that T5
and T7 arrive at approx the same time and about 40 nsec
before the other triggers.  Thus the coincidence triggers
normally take  priority in deciding the event type.  However,
one may look at the trigger bit pattern to see which triggers
coexist.

A ``trigger bit pattern'' in the datastream shows the
pattern of triggers which co-exist in an event.  This is
a TDC 1877 (multi-hit with up to 6 hits, a 5 microsec
window, one trigger on each of 12 channels, each channel
a separate trigger) which shows on channels 1-12 the
trigger 1-12 AFTER prescaling.  So, if a T5 is accompanied
by a T3 which survived prescaling, one would see a signal
on channels 3 & 5 with the appropriate timing.  For the
Hall A analyzer the data are parsed by the THaDecData
class, see db_D.dat in the database.

T2 and T4 are for measuring scintillator efficiency.
The definition has changed over time and at present
(Jan 2004) the definition requires 2 out of 3 from
among S1, S2, and a 3rd detector.  On the R-arm the
3rd detector is the "OR" of the 10 gas Cerenkov PMTs.
On the L-arm the 3rd detector is the "AND" of the 2 PMTs
of the S0 scintillator.  But as I said, this has changed
over time.  Let this 3rd detector be called D3.  Then:

T2 = ((D3.and.S1) .or. (D3.and.S2)) .and.  (.not.T1)

and similarly T4.

Typically the raw rate of T2(T4) should be 5% of T1(T3).
After cleanup cuts that require a track, etc, the T2(T4)
may be 2% of T1(T3), indicating a 1% scintillator inefficiency.
These are ballbpark expectations; if you don't see it, then
you have a problem.

WARNING !!!  WARNING !!  WARNING !!  :
The "CODA event type" is NOT NECESSARILY the "trigger type".
This is because of trigger supervisor functionality.
We try to arrange it such that the CODA event type is indeed
the trigger type, but the most fundamental way to sort out
what trigger type is to look at the trigger bit pattern explained
above.  Here is how the CODA event type is defined:  If trigger N
arrives first and survives prescaling then the CODA event type is N.
However, if triggers N and M overlap within 10 nsec and both survive
prescaling, then it is defined as event type 5.  (In former times this
was defined as event type 14, but that proved dangerously confusing.)
Ideally all triggers would be split apart enough that there would be
no such ambiguities, but this is difficult to arrange for other reasons.
So, in practice T5 is arranged to arrive first and will make CODA
event type 5, but if T5 is prescaled away then T1 and T3 may overlap
at the TS (especially for EDTM pulser signals) which again makes an
event type 5.  I realize this is confusing, but if you rely on the
trigger bit pattern, you can sort things out.

One reason the trigger bit pattern is important is that it
affects calculation of deadtime.  As I write this, the online
calculation does not know how many triggers of each type were
accepted, so we estimate the deadtime for online display (datamon
and end-of-run halog summary) by accounting for the correlations:
Every T5 is also a T1 and a T3, and every T7 is also a T6 and T5.
At present we don't account for the correlation of T3 and T6
which would be hard to do since it is neither 1.0 nor 0.0.
I will try to fix the online display before the experiment.
Offline, however, it is easy to calculate deadtime.

How the coincidence time is measured, and some explanation of
the glitchy features seen there, are explained in this document:
ctime.ps (postscript).

Electronics deadtime is a complicated issue.  There can be losses
in the trigger electronics, but also smearing of distributions
(TDCs, etc), and pileup in VDCs.  To provide a handle on some
of these things we have implemented a scheme to measure the

A general intro to the trigger complete with circuit diagrams can
be found at hallaweb.jlab.org/equipment/daq/trigger.html.