Cabling for DVI, Firewire, and USB 2.0 is paramount for each format in order to provide you with the performance specified. Recall that DVI (Digital Visual Interface) is penetrating the computer-monitor interface market as flat panel LCD monitors become affordable.

Firewire, or IEEE-1394, is that tiny, square-like connector tucked away on the side of your digital camcorder that allows you to upload DV format to your computer, among other things. And as we approach the real beginning of the millennium (2001), USB is receiving a major overhaul… analogous to jacking up your radiator cap and driving in a new car underneath it. Yes, USB 2.0 promises to bring us hot swappable, hosted peripherals now capable of talking at 480 Mbps instead of just 12 Mbps.

Getting From Here To There With DVI

The DVI (Digital Visual Interface) connection between local monitors and computers presents an interesting interfacing environment. It is a combination serial digital interface and a parallel interface format, somewhat like combining the broadcast serial digital and parallel digital interfaces.

Transmission of the TMDS (transition minimized differential signaling) format combines four differential, high-speed serial connections (in its base configuration) transmitted in a parallel bundle. When the DVI specification is extended to the dual mode operation, greater data rates for higher display resolutions are possible, but now there are seven parallel differential, high-speed pairs. Cabling and connection become extremely important. In this way, DVI is similar to the original D1 parallel interface which requires eight or ten differentially driven serial lines capable of handling a full byte on each clock cycle. If you have the opportunity, take a look at available D1 cables, and you will find them limited in usable lengths—very much like DVI.

The nominal DVI cable length limit is 4.6 meters (about 15 feet). Electrical performance requirements are similar to serial digital. Signal rise time (0.330 nanoseconds), cable impedance (100 ohms), far end crosstalk (FEXT) of no more than 5%, and signal rise time degradation (160 picoseconds maximum) are the key parameters highlighted in the DVI specification regarding the physical connection. Cable for DVI is application specific because maintaining these specifications is no easy feat since the actual bit rate per channel is 1.65 Gbps. And, we're talking twisted pair cable here.

Those of you familiar with CAT 5, CAT 5e, CAT 6, and CAT 7 (I feel like someone with too many cats), know the importance of cable and installation quality in order to meet performance. For CAT 6, the industry is talking 1 Gbps over four twisted pair wires over a distance of 100 meters. Sound something like DVI in terms of pairs and speed? And, the trend is to push for faster communication speeds. This makes these methods very similar in speed with DVI, but that's where the similarity ends. In high-speed data communications systems, there is significant overhead added to handle error correction. And, if some data is lost, it can be re-sent. With digital video interfaces like DVI, there is some error correction facility, but the delivery is a one-way street. If you fail to receive all the data bits required to make the system work, you lose picture information or lose the picture completely.

So, the DVI cable and its termination is very important. The physical parameters of the twisted pairs must be highly controlled. Specifications for the cable and the receiver are given in fractions of bit transmission time. Therefore, the requirements depend on the clock rate or signal resolution being used. Transferring the maximum rate (1600 x 1200 at 60 Hz) for a single link system means that one bit time (10 bits per pixel) is 0.1 (1/165 MHz), which is only 0.606 nanoseconds. Ten bit times describe one pixel in this system.

The DVI receiver specification allows only 0.40 x bit time, or about 0.242 nanoseconds intra-pair skew (within the twisted pair). Remember, this is differential transmission. The "eye" pattern seen at the receiver end must be as symmetrical as possible. Further, the inter-pair skew, which governs how bits will line up in time at the receiving decoder, may only be 0.6 x pixel time, or 3.64 nanoseconds. These parameters are largely responsible for the short transmission distances for DVI.

In addition to the above requirements, a cable for DVI should be evaluated on its insertion loss for a given length. The DVI transmitter output eye pattern is specified into a nominal cable impedance of 100 ohms. A normal signal swings +780 mV to -780 mV. The minimum positive signal swing is +200 mV and the minimum negative swing is -200 mV (total swing of 400 mV). When the signals are combined in the differential receiver, the resulting signal level is two times the swing value. But, for the cable situation, we must assume minimum performance on the transmitter side and best sensitivity on the receiver end. The receiver must operate on signals as low as +75 mV to -75 mV, or a total swing of 150 mV. This means that under worst-case conditions, the cable attenuation can be no more than 8.5dB at 1.65 GHz (10 bits/pixel times 165 MHz clock). As you can imagine, maintaining this type of performance on twisted pair wires is relatively difficult.

Figure 1 and Figure 2

DVI Connector - All For One and Two For All

Two versions of the connector emerged from its creator—the DDWG (Digital Display Working Group; more details and full specs at www.ddwg.org). The DDWG felt that the transitions from analog to digital monitor interfacing should be gradual with capability to support the analog VGA for some time. Therefore, there is a DVI-D (Figure 1) version for digital interfacing only and a DVI-I version (Figure 2), which contains both analog and digital interfaces. Neither version is like the earlier DFP connector.

DVI-D embodies 24 pins supporting the digital-only version. The combined digital and analog version, DVI-I, adds four additional segregated connections supporting analog RGB and horizontal sync, plus a fifth connection for ground. The combination connector is intended to transition product from analog to the fully digital connection over time. Pin arrangement in the digital portion of the connector supports logical arrangement of the differential pairs to support the high data rate. Although the DVI-I connector has many pins, it is not much larger than the current 15-pin HD VGA connector. Currently, termination of the connector is challenging due to the tooling and limited space within the assembly. The DVI connector is allowed a maximum of 0.160 nanoseconds rise time degradation to the signal.

Figure 3. IEEE 1394 4-pin

Figure 4. IEEE 1394 6-pin

Figure 5. IEEE 1394 Cable

DV and Firewire - Serial Digital for the Rest of Us

The new DV, or Digital Video, recording standard now driving most consumer camcorder purchases, is a serial digital format of 25 Mbps, sometimes called DV25. The Firewire (IEEE 1394) interface conveniently handles the data rate of DV, and then some. The DV format is the first application making tremendous use of the IEEE 1394 capability. IEEE 1394 is much bigger than DV in terms of data handling. This specification supports up to 400 Mbps currently and extensions to the standard are under consideration. Its key strengths are its "just-in-time" data delivery and peer-to-peer relationship…meaning that Firewire appliances can communicate without need for a host controller.

So, when we talk DV, we are really talking about using 1394 (I'm tired of typing IEEE) and a portion of its capability. The connection scheme and cabling for this interface are specific as well. The 1394 system utilizes two shielded twisted pairs and two single wires. The twisted pairs handle differential data and strobe (assists in clock regeneration) while the separate wires provide power and ground for remote devices needing power support. Signal level is 265 mV differential into 110 ohms.

The 1394 specification limits cable length to 4.5 meters in order to satisfy the round trip time maximum required by the arbitration protocol. Some applications may run longer lengths when the data rate is lowered to the 100 Mbps level. The typical cable has 28 gauge copper twisted pairs and 22 gauge wires for power and ground. A Firewire connected appliance may or may not need power from its host, but must be capable of providing limited power for downstream devices. The 1394 specification supports two plug configurations—a four-pin version (Figure 3) and a six-pin version (Figure 4). Six-pin versions can carry all six connections and are capable of providing power to appliances that need it. For independently powered appliances, like camcorders, the four-pin version is used for its compactness. Cable assemblies have the data signal pairs crossed over to avoid polarity issues. All 1394 type appliances have receptacles, which makes for easy upstream-downstream connection with the male-to-male cable.

The 1394 specification provides electrical performance requirements, which leave open the actual parameters of the cable design. As with all differential signaling systems, pair-to-pair data skew is critical…<0.40 nanoseconds. Crosstalk must be maintained below -26 dB from 1 to 500 MHz. The only requirement on the size of wire used is that velocity of propagation must not exceed 5.05 nS/meter. Refer to Table 1 for other critical details of the physical interface system for 1394. Figure 5 shows the cable internal conductor arrangement.

Figure 6. USB "A" and "B" connectors

USB 2.0 - Fire In Another Wire

The USB, universal serial bus, simplifies connection of computer peripherals. USB 1.1 is limited to a communications rate of 12 Mbps, which is plenty fast for most items like printers, audio devices, keyboards, scanners, etc. During 1999 the USB Implementers Forum began work to upgrade USB capability by more than 40 times. The new USB 2.0 interface will support up to 480 Mbps communication. It is anticipated that USB 2.0 can replace higher cost SCSI interfaces for some peripherals. In-depth information is available at www.usb.org.

The Implementers Forum says that fully compliant USB 1.1 cables will perform at USB 2.0 speeds. USB cables utilize two specially designed 4-pin plugs and receptacles. The "upstream" plug is called "A" and the "downstream" plug is called "B" (see Figure 6). This format is intended to minimize end user termination problems, thereby ensuring proper connectivity. Use the A connector to connect with a host or downstream connection on a hub. Use the B connector to connect to the peripheral appliance.

Table 1. Critical IEEE 1394 Timing Parameters

The USB cable consists of one twisted pair for data and two untwisted wires for powering downstream appliances. Specifically, a full-speed cable contains a 28-gauge twisted pair, an untwisted pair of 28 to 20 gauge power conductors, an aluminized polyester shield, a drain wire, and an overall 65% (minimum) copper braid. Nominal impedance for the data pair is 90 ohms. The maximum cable length for USB is a function of signal propagation delay. The cable may have no more than 26 nS delay from connector A to connector B. An additional allowance of 4 nS is split between the sending device connection and the receiver connection/response function, making the entire one-way delay 30 nS maximum. In addition, the cable may not have a velocity of propagation greater than 5.2 nS per meter. The length and twist of the data pair must be matched well enough so that no more than 0.10 nS time skew exists between bit polarities. The nominal differential signal level is 800 mV.

The digital video and data world is exciting, but, as you can see, assembling high-speed data cables is not going to be a trivial or casual task. Why, I've just gotten the hang of crimping on BNC connectors. What do you think about USB on BNC? You need power? Well, with some external wires and a little Scotch tape, we can…