Max number of channels in a 4.2 rig confusion?

Okay, I am about to probably make an idiot out of myself with this one, but for the life of me I can NOT figure out from the knowledge base WHY they list 72 as the max number of channels.. I understand that the standard Disk I/O cards get 8 a piece so that should be 24 IO pairs right? Then they list an additional 5 Bridge IO cards with 8 a piece there as well so 40 more.. Now that would give a total of 64 pair IO.. Not 72. They then list 4 Farm cards, which is okay, but still no more inputs right? They are farms.. So where does the magic 8 extra inputs come from?? Some of the Digital IOs or something on the Bridge cards? Or on the interfaces on the Disk I/O cards???? This is nubus with the expansion chassis. As one last question, am I under the correct assumption that no matter what even with 72 IO's I should only be able to record 24 of them at any given time which is the limitation of the 3 disk IO setup under NUBUS TDM???? I am new to a lot of this, but the math just doesn't seem to come out right regardless. ppuskari2

Anyone from Digi have an answer on this one? I've combed through the manuals and such, and am still stumped with how 72 channels is the limit and not 64... Thanks in advance. If I can find the cards, I might just try and build a maxxed out unit. Currently have: 3 Disk I/O 1 Bridge I/O 4 Farms 2 Sample Cell II TDM 1 Lexicon NUVerb TDM One blank slot in the 12 slot Nubus chassis... So that should be giving me 32 channels of I/O Will be upgrading to 4.x or above very shortly in my old school Quadra 650 with maxxed ram and a 4 gig hd 040 ECI card for chassis ppuskari2

Pro Tools software had a limit of 72 channels of I/O, but also had a limits as far as how many 'interface' cards were supported. The example in ab3825 lists 3 Disk I/Os and 5 Bridge I/Os (all 8ch NuBus cards) for a total of 64 channels of I/O. I'm pretty sure that was the limit with PTIII NuBus (ab25018 lists 7 'interface' cards, but that's for PTIII-PCI). I'd have to track down a 4.x Expanded System Guide to confirm. I will see that ab3825 is updated a bit to avoid confusion.