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A DisplayPort signal can be multiplexed onto a Thunderbolt cable. In fact, multiple DisplayPort signals (Intel Cactus Ridge 4C) can be multiplexed onto the same Thunderbolt cable.

Multiple Thunderbolt monitors may be daisy-chained on the same Thunderbolt cable, provided that the graphics card in the PC supports this.

However, a passive Thunderbolt to DisplayPort adapter may only be connected at the end of a Thunderbolt daisy-chain.

I'm confused as to how Thunderbolt monitors can be daisy-chained in light of the restriction on passive DisplayPort adapters above. Does each Thunderbolt monitor contain its own PCIe graphics card? Is there a special Thunderbolt to DisplayPort chip that can be daisy-chained to other devices?

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  • I thought thunderbolt was its own spec – Voltage Spike Mar 7 '17 at 18:14
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Thunderbolt 3 is a protocol based around Intel's Alpine Ridge controlles. These controllers take PCIe and DisplayPort signals and multiplex them down the same cable using the Thunderbolt 3 protocol.

For active Thunderbolt 3 devices, the AR controller acts as the demultiplexer whist also handling daisy chaining (each link is point to point and the controller routes packets between the two links). The demultiplexer can be used to isolate the DisplayPort data which can then be used by the display.

Passive adapters must be at the end of the chain because they have no AR controller and so lack the smarts to daisy chain the bus. To facilitate the passive adapters, Thunderbolt 3 has an alternate mode in which raw DisplayPort signals are sent down the cable. Once a thunderbolt link is in the alternate mode, further thunderbolt devices can't be connected as the signalling is no longer Thunderbolt.

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  • So each Thunderbolt monitor has an AR controller. The first one isolates one DisplayPort signal and uses it for its own display. The second monitor's AR controller isolates the other DisplayPort signal and passes the rest of the Thunderbolt data down the line. Do I have this right? – watkipet Mar 7 '17 at 22:40
  • @watkipet pretty much. Multiple monitors via thunderbolt only works if the graphics card which is connected to the AR controller can support multiple displays on one displayport stream. – Tom Carpenter Mar 7 '17 at 22:56
  • One DisplayPort stream? The example I'm thinking of is the Mid 2012 MacBook Air. It can drive two daisy-chained Thunderbolt monitors, the Intel HD 4000 graphics card supports 3 displays, and its Cactus Ridge 4C Thunderbolt controller supports 2 DisplayPort outputs. However, the Intel HD 4000 doesn't support DisplayPort 1.2 and thus cannot do DisplayPort daisy chaining--yet that Mac can driver two daisy-chained Thunderbolt monitors. For that example, do you think there's one DisplayPort stream or two? – watkipet Mar 7 '17 at 23:05
  • @watkipet my bad, two streams, one port. Per the Thunderbolt 3 FAQ: "Thunderbolt 3 is based on the DisplayPort 1.2 specification and can support up to 2 streams (eight lanes) of DisplayPort 1.2 video bandwidth" – Tom Carpenter Mar 7 '17 at 23:08
  • Thanks! I'm surprised then, that a vendor doesn't offer a Cactus Ridge 4C, Eagle Ridge, or Alpine Ridge device that takes a Thunderbolt Signal in, and provides Thunderbolt and DisplayPort outputs. Then users could buy two such dongles, daisy chain them, and connect two inexpensive DisplayPort monitors to laptops such as the mid 2012 MacBook Air that I mentioned. – watkipet Mar 7 '17 at 23:16

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