Comment by wtallis | Hacker News Reader

Comment by wtallis | Hacker News Reader

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A current Thunderbolt port provides a DisplayPort 1.1a connection, which has 8.64Gbps. A 30" monitor currently means 2560x1600 at either 24bpp or (more commonly) 30bpp. Even at 24bpp and using CVT-RB, that requires 6.45 Gbps per display. The MacBook Pro only supports two 30" monitors by having them connected to two separate Thunderbolt ports, rather than daisy-chained as is possible with lower resolutions.

EDIT: It looks like the provisioning of bandwidth for a Thunderbolt connection is more complicated, and it's hard to tell from Wikipedia exactly how it works. Thunderbolt and DisplayPort both support 4 high-speed lanes. In Thunderbolt, the lanes are currently 10Gbps, configured as two pairs of input and output lanes carrying PCIe traffic. In DP, they're 4 outbound lanes at up to 5.4 Gbps per lane (for DP 1.2). It appears that Thunderbolt, despite not yet supporting DP 1.2, can operate in DP mode at DP 1.2 speeds of 5.4Gbps per lane, but it's not clear if Thunderbolt supports the multiple stream transport (MST) feature of DP 1.2 that is necessary to drive more than one display over a single DP connection. If a TB cable is configured to provide two DP connections of two lanes each, then it would work, but then it seems that further daisy-chaining of TB devices wouldn't work. If Thunderbolt can drive a monitor with all lanes configured to carry PCIe traffic, that's news to me, and I want to know how Intel and Apple pulled that off.

A MacBook air with 1 current generation thunderbolt port can drive two 2560x1440 thunderbolt displays via daisy chaining.

I can't find any indication on Apple's site that the Air can drive more than one of Apple's Thunderbolt displays, and the math doesn't support your assertion that it can: two such displays would require a minimum of 11.6 Gbps, which can only be provided by DisplayPort version 1.2 (or later). EDIT: If the video data is being sent with the cable operating in pure Thunderbolt mode with no DisplayPort lanes configured for the connection, then there's enough bandwidth, but I don't know how that would work.

All of Apple's 2012 models (save the Mac Pro), and a few of its 2011 models, can drive 2 Thunderbolt displays:

http://support.apple.com/kb/HT5219?viewlocale=en_US#dispnum

I've done this myself with both the MacBook Air (Mid 2012) and the Mac Mini (Late 2012).

A single current generation Thunderbolt port runs two separate 10Gbit channels. Each channel is a mux of PCI-E data and a DisplayPort 1.1a stream. Two displays (each up to DisplayPort 1.1a's 8.64Gb maximum bandwidth) can be driven on one chain, one off of each channel. This can either be two Thunderbolt displays or one Thunderbolt display and one pass through raw DisplayPort display at the end.

Some restrictions apply due to the need for one Thunderbolt controller chip per display to do the DisplayPort demuxing. A Thunderbolt monitor will typically have one Thunderbolt chip and will use its DisplayPort output itself. A DisplayPort display can't be daisy chained directly after a Thunderbolt display (but can after a non-display Thunderbolt peripheral) for this reason.

It sounds like (according to the OP article) that the 2013 Thunderbolt chips (Redwood Ridge) will allow DisplayPort 1.2 pass through only. In this configuration the port could either be run as a Thunderbolt chain (with the above limitations to 2xDP 1.1) or as a DisplayPort 1.2 port. In 1.2 mode Thunderbolt peripherals couldn't be used, but DisplayPort 1.2 features (in particular 4k support and MST) could be used. Presumably the 2014 20Gbit channel Thunderbolt could allow 2xDP 1.2 as that tops out at 17.28Gbit.

Edit: yes, this means that devices sharing the chain with two Thunderbolt displays will have constrained incoming bandwidth. Probably even more so as the only Thunderbolt displays in existence also sport USB, speakers, a webcam, FW800, and Gigabit Ethernet, which together could easily eat up the remaining bandwidth on the TB channel. This may also be part of the reason why Apple hasn't released an updated Thunderbolt display with USB3, 2x10Gbit doesn't go very far with a pair of 27 inch displays hogging most of it.

Ok. That clears things up a bit. So a PCIe device and a DP device can share the same 10Gb channel at the same time? Does this mean that the presence of a DP device will mean that writes to the PCIe device are much slower than the corresponding reads (since the DP device is only using bandwidth in one direction)?

A video of an MBP (not an air) doing this is here:

http://www.youtube.com/watch?v=sP1thwUcO9c

Skip to 30s in to see the single TB cable driving the two displays. You really can drive two of these monitors thru a single TB connection.

> Thunderbolt can currently drive only one 30" monitor.

The 27" monitors are not 30" monitors.

The 30" quoted by the GP was 2560x1600 pixels

The TB 27" monitor has 2560x1440 pixels (10% less)

That's close enough for the ballpark calculation of the GP.

(I'm not saying I 100% follow the GP's understanding of the numbers and systems issues, because I don't. I responded because I've seen the two-monitor, one-cable setup in practice.)

You're right. I was thinking of the similar-but-crucially-different situation of using two Apple Thunderbolt Displays chained together. Multiple sources confirm that works, but obviously that doesn't help if you're trying to use a pair of non-Thunderbolt DisplayPort monitors. The combination of slightly lower resolution (1440 vs 1600) and the built-in controller seems to make all the difference.

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