iMac i9 vs iMac Pro Thunderbolt Busses

Discussion in 'iMac' started by ridgero, Jun 8, 2019.

  1. ridgero macrumors member


    Dec 16, 2017
    Hey there,

    I don't have an iMac yet, but I going to buy one later this year.

    iMac i9 has 1 Bus with 2 TB3
    iMac Pro has 2 Busses with 4 TB3

    Is there any guide about the maximum connection on one TB3 Bus?

    I am planning to add another screen (2560 × 1440p) and 2 external SSDs.

  2. iNewbie macrumors regular

    Aug 25, 2006
    I'd be interested in hearing about this also.
  3. SecuritySteve macrumors 6502a


    Jul 6, 2017
    The maximum speed of one TB3 Bus is 40 GB/s.

    A monitor running 2560x1440 resolution at 60 Hz requires ~6.64 GB/s bandwidth.

    Assuming your SSDs were running with 3 GB/s read / writes (NVMe speeds) you still have plenty of overhead with a 2560x1440 monitor.

    You can do the math yourself:

    2560x1440 = 3,686,400 pixels total

    3686400x60 Frames per second (Hz) = 221,184,000 Bytes / Second, or ~221 MB/s

    221 MB x 10 bits / clock (This is a clock coefficient) x 3 channels = 6.64 GB/s bandwidth.
  4. mikehalloran macrumors 65816

    Oct 14, 2018
    The Sillie Con Valley
    Where this becomes important is what monitors you plan to hook up and how. Apple has support bulletins on this. This is one of many and contains links to the others. Long story short: iMP supports two external 5K monitors, one on each bus. 2017/2019 iMac supports one unless you buy an external GPU.

    Otherwise, the iMac Pro has two TB3 busses while all other TB3 Macs have one.

    I have two 4K monitors connected via USB-C through USB-C–DP cables plus a pair of drives sitting in a TB2 dock connected through the TB2–TB3 adapter. That dock is in the middle of a daisy-chain between the adapter and my FireWire audio interface through the FW–TB adapter. This leaves one port free.

    Although that takes up three USB-C ports, I need only one bus. If I didn't have an iMac Pro, I'd need a USB-C hub for the two monitors or a TB3 hub for all three peripherals.

    If that seems a little confusing, yes it is.

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3 June 8, 2019