IEEE 1394 Bus Galvanic Isolation Issues.
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.
The CE equipment side of the diagram consists of typical consumer electronic gear in a home entertainment cluster. Each of the connected devices is equipped with a two-wire AC power cord. Invisible to the user, however, are the grounds ("A" and "B") introduced by the antenna and the CATV hook-up in the system. Assuming these two grounds are located close to each other, this bus system should also operate well. In the case of antenna and CATV grounds being located at some distance from each-other, proper operation of the bus will depend upon the amount of potential differential and associated current between the CATV box and the DVCR.
CE Meets PC
Finally, when we interconnect the CE side with the PC side of the diagram, the third ground ("C") is introduced into the system. The more grounded devices we interconnect, the greater the possibility of producing electrical noise currents in the bus ground system. To make this point more clearly, if we consider a typical single family home in suburbia, the three grounds shown in the Fig will probably be no more than a few (maybe 25) feet apart. Ground noise voltages on the order of a hundred millivolts will be seen on the cable grounds, mostly due to the switching power supplies in the PC equipment. All is well in suburbia.
Now, let's take this arrangement and move it to the city, to an older high rise building, for example. The grounding points may be physically located several hundred feet from each other. The PC ground will have noise on it not only from its local switching power supply, but also from several hundred feet of common safety ground wiring within the building. The potential differences between the PC frame ground and the other grounds may be in the range of volts to tens of volts. When an elevator motor on the roof starts, there will be even more noise introduced to the building ground wiring network. Add to this the fact that the antenna is on the roof of the building and the CATV service enters in the basement and there's the potential for local ground point separations of hundreds of feet.
Interconnecting equipment in the city environment is not the only area of concern. Commercial and industrial use of the IEEE 1394 bus is another source of possible problems. These applications typically have longer ground wiring runs and more noise generating equipment interconnected. CATV and antenna wiring entry points may also be great distances apart.
What's the effect of not providing isolation when required? As stated above, the bus communication could cease or, worse yet, become unreliable. Unreliable communication is typically worse than total cessation. When total cessation occurs, the system stops working and the user immediately knows something has gone wrong. When the system stops working momentarily at odd intervals (maybe when the elevator motor on the roof starts), the user is unable to connect the outage event with any visible source of the problem.
How should designers cope with this problem? As mentioned previously, the IEEE 1394-1995 standard recommends that all nodes subject to an earth ground connection employ the use of galvanic isolation around the bus-driving component. Galvanic isolation takes two forms (per the 1394 standard): transformer and capacitive. Transformer isolation is best for use in the presence of large amounts of electrical noise (up to 500 volts of isolation). However, it is very costly and consumes a large amount of volume in the end product.
Capacitive isolation, capable of providing about 50 volts of isolation, is the more cost effective and space efficient solution. Diligent chip manufacturers have taken the recommended capacitive solution and reduced its complexity by integrating some of the external components into chips. This further reduces component count and, thus, saves space and money. Capacitive isolation is suitable for most applications in the PC and CE world. However, some industrial and commercial applications may require isolation voltage levels in excess of what the capacitive solution is capable of providing. Transformer isolation should, therefore, be used in these applications.
Galvanic isolation acts to isolate the PHY chip (this chip is the bus transceiver, it transmits and receives all of the signals on the bus) from the rest of the subsystem in which it resides. By isolating the PHY chip, there is no direct (low resistance) connection of the local ground to the bus ground. Instead, there is a high resistance connection in the case of capacitive isolation or there is no connection at all in the case of the employment of transformer isolation. If a high resistance connection is used, there may still be some noise current flow on the bus ground line between nodes, but it is typically on the order of a milliampere of current. This amount of current flow is far below the level that would cause problems.
Yet isolating the PHY chip alone does not totally isolate the node. Accompanying the PHY isolation, the power supply for the PHY chip must also be isolated. Typically, a transformer (or an additional transformer winding) is used. With the PHY chip isolated, the ground potential at which the PHY operates is the same as the ground potential provided by the opposite connected node. Therefore, since the potentials are equal, no bus ground noise current flows.
If both connected nodes are equipped with isolation, the bus ground potential is brought to a voltage level, which is between the voltage levels present on the grounds of the connected devices. That is to say, connecting an isolated PC interface card to an isolated DVCR results in no bus ground noise current flow and a voltage level on the bus ground that is about 1/2 of the total voltage difference between the PC frame ground and the antenna ground. This isolated node-to-isolated node interface is maximally safe from ground noise current flow and can withstand higher noise voltage levels than either node being used opposite a grounded node.
Isolation does not come free of cost. Capacitively isolating a 400Mbit/sec capable node costs about $3.00. This translates to about $12.00 additional end user cost. Compared to the cost of troubleshooting an isolation problem and/or replacing a piece of equipment, this cost can be seen as quite reasonable. The use of transformer isolation increases the cost of isolation to about $32.00, which translates to over $120.00 added cost to the end user.
The IEEE 1394 standard recommends that all nodes subject to connection to an earth ground employ galvanic isolation. Not all companies producing 1394 bus connected equipment use galvanic isolation. Some reason that they cannot foresee any node on the bus being connected to an earth ground. Many CE manufacturers have learned to play it safe by providing isolation in their equipment. It is incumbent on the system integrator or designer to select equipment, which will properly interface with all the nodes on the bus given a specific bus architecture.
The future holds not only higher bus speeds, but also the use of optical fiber in place of the present-day copper conductors. When optical fiber is employed, the need for galvanic isolation will diminish or completely disappear. Until then, employing isolation is the safest way to guarantee. proper IEEE 1394 bus performance.
Bob Parker is an IEEE 1394 applications engineer at Philips Semiconductors (Sunnyvale, CA).
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|Title Annotation:||Technology Information; proposed standard|
|Publication:||Computer Technology Review|
|Date:||Sep 1, 1999|
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