DisplayPort Technology — Heir Apparent to VGA?
- Created: Thursday, 01 November 2007
For the last 20 years we have connected our monitors to our computer systems having the confidence that upon turn-on, we could surf or create with never a consideration of whether the display would operate satisfactorily. VGA (Video Graphics Adaptor) technology enabled us to do that with CRTs as well as with flat panel displays. Now, however, the viewing benchmarks are being reset, as are system design considerations. There’s a new interface on the horizon called DisplayPort!
DisplayPort is being administered under the auspices of VESA (Video Electronics Standards Association) and is directly targeted for digital video content to PC monitors. The interface was released in May of 2006 and its second revision was released earlier this year. It has big names behind it such as Dell, HP, and Intel, and the pace of the rollout, as well as the commitment from IC vendors, herald DisplayPort’s potential success as a replacement for VGA. With respect to the “unconsciousness interoperability” that VGA provided, you can rest assured that VESA is working hard to maintain that reality with DisplayPort.
You might remember that DVI (Digital Visual Interface) offered similar promise a number of years ago, but for various reasons never really took off. DVI even implemented a viable transition strategy with its DVI-I connector that included VGA capability directly with its digital interface. The connector, three times the cross sectional area of the DisplayPort connector, was uninspiring and DVI quite simply languished. Size not withstanding, what are the other consumer recognizable benefits of DisplayPort? Aside from the push-in connector with latching mechanism there really isn’t much else one can directly see. There is, however, a number of points when you consider what is enabled by the DisplayPort technology.
The DisplayPort interface is integrally composed of 4 data lanes, 1 auxiliary channel, and a hot plug detect (HPD). The data lanes (Main Link) are unidirectional from the source to the sink, 100 ohm differential, and can operate at either 1.62 or 2.7 Gbs. The auxiliary channel (AUX) is a lower rate (1Mbs) communication path between a DisplayPort source and a DisplayPort sink where the source is the master. The receiver can initiate service from the source by asserting the HPD line. The specification defines four different Main Link levels (400, 600, 800 and 1200 mVolts pk-pk) as well as four possible pre-emphasis settings for these lanes (0, 3.5, 6, and 9.5 dB). Further, to complete the ultimate configurability of the interface, the specification allows for one, two, or four lane operation. The interface supports not only box-to-box but an embedded implementation model as well.
Benefits of DisplayPort
Consider, a source/sink connection that is marginal or worse. What system today could handle the situation without user interaction? The configurability of the main link and the link communication path (Aux channel) makes it clear that DisplayPort is a very dynamic interface that will adapt as necessary to optimize the link. For the user this usually means to choose the highest resolution possible, and thus best viewing quality possible for his/her system. For the system designer, it may mean the fewest number of wires from source to sink, or the least amount of power, the least expensive cable, or the longest cable. For our example of a marginal setup, the DisplayPort sink could assert the HPD line and, when queried, could recommend to the DisplayPort source a higher transmitted voltage, more preemphasis, or lower data rate.
For PC manufacturers, DisplayPort enables a concept called “Direct Drive” which merely means that the display module can be directly connected to the cable input without having to go through a TMDS (Transition Minimized Differential Signaling. Used in HDMI and DVI systems.) receiver or a scalar device. In addition, DisplayPort can be implemented on the latest IC processes (65nm) because it allows for a lower bias voltage. The implication here is that motherboard pin count, IC count, and power requirements are reduced.