IEEE 1394 Bus Galvanic Isolation Issues - proposed standard - Technology Information

Computer Technology Review, Sept, 1999 by Bob Parker

The IEEE 1394 serial bus standard is gaining significant support as the high-speed link of choice between the Personal Computer (PC) and Consumer Electronic (CE) worlds. This bus currently supports speeds of 100 to 400Mbit/sec and will allow future speeds up to 3.2Gbit/sec. The fact that it is plug-and-play and the bus architecture is peer to peer (does not require a "master" node) make adoption of the bus easy for both the manufacturer and the end user. However, without appropriate consideration given to grounding issues, designers risk delivering products that may be unreliable or may not work at all.

It is recommended by the IEEE 1394-1995 standard that all nodes subject to an earth ground connection should employ the use of galvanic isolation around the component that drives the bus (the physical layer control chip or PHY). Galvanic isolation refers to the isolation of bus signals from power line ground noise currents. These currents can adversely affect the operation of the bus, rendering it nonfunctional or, often worse, unreliable. Unfortunately, not all manufacturers consistently implement galvanic isolation due to cost or a lack of understanding of the need. Additionally, designers may not realize that their application will be subjected to an earth ground situation. In these instances the recommendation fails to protect the end user.

Three Wires Meet Two

The PC world consists of three wire power cords and grounded equipment cases. Only laptop computers have thus far graduated to the two-wire world of consumer products. Where two wire and three wire worlds meet, there is the possibility of ground noise current problems. If they meet via the IEEE 1394 bus, the designer must take certain precautions or else the potential for problems increases.

PCs have historically utilized switching power supplies for obtaining the low level dc voltages required by logic chips. The switching power supplies, while efficient and small, generate electrical noise. In order to get rid of this electrical switching noise, power supplies use filter networks on their ac power lines. These networks provide cancellation of the switching noise currents on the power lines. Resultant "left-over" noise currents are sent down the third "safety ground" wire to an earth ground at some distance from the PC's power outlet. The use of the safety ground wire in this manner is problematic due to the introduction of noise currents to the supposedly "quiet" ground path. Additionally, there may be differences of potential between various safety grounds that are connected to various devices along the IEEE 1394 bus.

The Fig shows a sample home IEEE 1394 bus interconnection diagram. Note that there is not just one earth ground connected to the system. Ground "C" at the PC workstation is a readily apparent connection to ground due to the use of three-wire power cords. The other two ground connections ("A" and "B") may not be obvious to the user and, worse, may not have been considered by the designer. Electrical codes in most countries require a connection to local earth ground for antenna wiring and distributed CATV wiring. The connections to ground are safety-related mechanisms for the elimination of electrostatic voltage buildup and the safe conduction of lightning-induced currents to earth ground. Due to the distances between the system grounds, there can be significant differences of potential between the local grounding points. The voltages produced are typically on the order of millivolts to a volt, but can be as much as 10 or 20 volts if the ground points are separated by hundreds of feet. The potential exists for high currents (amperes) to flow when the grounds are interconnected because the earth is a large body, low impedance source.

The IEEE 1394 bus sends data between various nodes on pairs of wires within the bus cable. The signaling technique used is called differential signaling; this type of signaling is very resistant to the influences of outside electrical noise. However, other signals transmitted along the 1394 bus (speed, connection, and bias) are not sent in differential mode, but rather in common mode. These signals rely on the ground reference wire in the cable for proper operation. This is where the potential for interference from ground-conducted electrical noise is the greatest. Minor (short-term) interference with any of the common mode signals can cause communication on the bus to become unreliable. Continuous or low frequency interference with the connection signal or the bias signal can cause the entire bus to stop working.

If we consider the PC workstation side of the Fig on its own, the computer peripherals are connected to one AC outlet (or multi-outlet strip). The ground ("C") introduced to the system is local and all connections are made over relatively short wires. The typical voltage differences of individual device grounds are in the range of a few millivolts. If the 1394 bus cables are each connected to the peripheral device ground, there is probably only a few milliamps of noise current flowing in the cable ground wires between peripherals (nodes) on the 1394 bus. This part of the bus should operate well while disconnected from the CE equipment side.