Daisy-Chaining Not Possible with mini-DP and Apple’s Thunderbolt Displays

We posted last week about the strong support that Thunderbolt received at Intel’s latest Developer Forum, so it seems appropriate that Apple has just begun shipping its first 27″ Thunderbolt displays to its customers. With this, Apple updated its support site and at the same time made a couple of limitations public – limitations that would have been nice to know about long ago.

As MacRumors reports, both the MacBook Air and MacBook Pro have the most apparent limitations. On the Air, just a single Thunderbolt display can be used, and on the MacBook Pro, using 2x Thunderbolt displays will render the notebook screen itself inoperable. Neither of these limitations seem that severe, as I’d guess few people with an Air would be looking to pair up 2x 27″ displays, and it can be assumed that if someone is running 2x 27″ displays with a MacBook Pro, they are not likely going to miss the 13″. This does prove that not all computers that have Thunderbolt will have the same capabilities, however.

On the mid and late 2011 iMac, 2x Thunderbolt displays are supported just fine, made easier by the fact that the iMac itself includes two ports. On the mid 2011 Mac mini, 2x are still supported, but since the device has just one port, both monitors will have to be connected to each other, via another Thunderbolt cable.

The greatest limitation that came about with the support page update is that a mini-DisplayPort Cinema display cannot be daisy-chained to one of the new Thunderbolt displays. As Thunderbolt is in essence DisplayPort+PCIe Bus, it was widely believed that daisy-chaining would not be an issue. As a mini-DisplayPort monitor can be plugged into a Thunderbolt connector without issue, the fact that a mini-DisplayPort monitor can’t be plugged into the Thunderbolt port on the latest monitor is a little confusing.

Interestingly, Macworld did some tests and discovered that if a Thunderbolt peripheral is placed in the sequence of ‘Thunderbolt display > PROMISE Thunderbolt NAS > Cinema mini-DisplayPort display’, both monitors will function just fine. The fact that this works confuses the situation even more, because if the mini-DP display works when plugged into a Thunderbolt peripheral, then plugging the display straight into a new Thunderbolt display should work. This seems to be a hardware limitation by Apple, not a limitation with the Thunderbolt spec itself, which does support daisy-chaining of multiple monitors.

Whether or not a fix will be issued is uncertain. Given the fact that Apple notes these limitations in its support pages, it seems likely that this is not an issue that can be fixed via firmware, though it is possible. It seems likely that future Thunderbolt displays from Apple will not have this limitation, unless Apple did this on purpose to avoid some issue we’re not yet aware of.