Overall trigger timing

Timing Scheme

From Elliott: Below is my current understanding of the trigger timing for the BCAL test:

Time	What	Comment
0 ns	electron hits tagger foil
115 ns	photon hits BCAL	125 ns to alcove, but BCAL is about 10 feet upstream on the platform
215 ns	BCAL PMT signals reach discriminators	20 ns in PMT plus 80 ns cable
245 ns	discriminator sum outputs available	30 ns a guess, but must be delayed before final AND in any case
310 ns	Tagger MOR reaches final AND logic	230 ns + 80 ns? cable, see Tagger trigger timing
310 ns	discriminator sums reach final AND	Delayed to match MOR
325 ns	result of final AND reaches trigger supervisor	15 ns a guess through trigger logic
370 ns	trigger supervisor L1 accept ready	45 ns internal to TS
385 ns	gate reaches ADC crate	15 ns a guess, cable plus gate generator plus cable
385 ns	BCAL signals reach ADC crate	20 ns in PMT plus 250 ns cable plus photon flight time

Note that my calculation shows the ADC gate reaching the crate EXACTLY at the same time as the signals. This is not a good situation as any slip in timing cuts off the ADC. But also note that the 80 ns cable from the CLAS area to our crates is also a guess, perhaps 60 ns is correct. Even so this only gives us 20 ns to play with.

Thus I tentatively conclude the current cabling will NOT work.

Two solutions are possible: shorten the MOR delay, or add delay to the ADC signals. The former is far preferable as it only involves one cable, whereas the latter involves 36 cables. But it is not clear how much we can shorten the MOR via a direct fast cable between the tagger and the BCAL trigger crates.

I propose we proceed on the former. If the latter becomes necessary there are a few possibilities, including purchase of cables, cutting some of the unused 170 ns in half and installing new ends to give 80 ns delay cables, etc.

Comments are welcome...

Comments

I agree that this will not work. I also agree on the plan of action, in the following priority: 1) Try to shorten MOR, 2) Cut unused 70ns cable (if there are enough and if this is ok with folks like Richard) and 3) Purchase new cables. -- Zisis