Apple's $50 Thunderbolt cable

It actually has more than wire inside. [Link]

The first Thunderbolt compatible peripherals—Promise's Pegasus RAIDs—starting shipping on Tuesday. Using the RAIDs with a Thunderbolt equipped Mac, though, requires a rather expensive $50 cable that is only available from Apple. We dug into the design of the cable to find out why Apple felt justified in charging $50 for some plastic-wrapped copper wire, and why Thunderbolt may have a hard time gaining traction outside of the higher-end storage and video device market—a fate similar to Apple's FireWire.

Promise's RAIDs do not come supplied with a Thunderbolt cable. Instead, users are directed to buy a Thunderbolt cable directly from Apple, which costs $49 for two-meter length. We contacted Promise to find out why a Mini DisplayPort cable could not be used in its stead, since the Thunderbolt port is based on Mini DisplayPort. A support technician told Ars that Apple's cable is a "smart" cable that "has firmware in it."

Intel confirmed that Thunderbolt requires specific Thunderbolt cables. "Only Thunderbolt cables can be used to connect Thunderbolt products using Thunderbolt connectors," Intel spokesperson Dave Salvator told Ars. "The cables have been designed for the 10Gbps signaling as well as power delivery that are part of Thunderbolt technology."

Active cabling required

Apple didn't respond to our requests for further information about the "firmware in the cable," but an EETimes article from earlier this year noted that in addition to having different electrical characteristics from Mini DisplayPort, Thunderbolt also uses active cabling to achieve full duplex 10Gbps transmission.

A source within the telecom industry explained to Ars that active cables are commonly used at data rates above 5Gbps. These cables contain tiny chips at either end that are calibrated to the attenuation and dispersion properties of the wire between them. Compensating for these properties "greatly improves the signal-to-noise ratio" for high-bandwidth data transmission.

Our friends at iFixit made a trip to a local Apple Store to find out what hardware powers Apple's Thunderbolt cable. CEO Kyle Wiens told Ars that Apple's cable contains two Gennum GN2033 Thunderbolt Transceiver chips to facilitate Thunderbolt's blazing speed.