Termination Explained

I’m going to try to explain termination, because FireWire uses termination as well as SCSI, but very few people really catch on as to how.

SCSI Termination

We all know that a SCSI chain needs to be terminated on the first and last device, but very few people realize where that first or last device is. Let’s take a chain as follows, a standard Macintosh Installation:

Internal Hard Drive <=> Built-in CD-ROM Drive <=> Internal SCSI Card (Built-in)

In this configuration, the hard drive needs to be terminated. The built-in SCSI serves as the terminator on the other side of the chain, since the SCSI card occupies SCSI ID 7 (some think this is a little known fact, that the card is device 7, but it always is).

By adding a scanner externally, we now have another issue.

(T)Hard drive <=> CD-ROM <=> SCSI Card <=> Scanner(T)

Several notes to add. This assumes that the Hard Drive is the physical last device on the internal SCSI cable. ID has nothing to do with termination power.

If we add an Iomega internal, and a SyQuest external, the chain would be as follows.

(T)Hard drive <=> Iomega <=> CD-ROM <=> SCSI Card <=> Scanner <=> SyQuest(T)

I’ve chosen to put the SyQuest on the end of the external chain and terminate it, simply because my SyQuest drive will be on all the time, where the scanner won’t be. Since my SyQuest provides it’s own termination power, I can know that if I turn on my scanner, there won’t be an issue. If I switched the scanner and SyQuest, it would be an unreliable SCSI chain when I turned the scanner on, because a device would exist beyond the auto terminated SyQuest drive. Scanner access under these conditions might or might not work depending on the sensitivity and stability of cables, other SCSI devices on the chain, and the built in SCSI controller itself.

Rules to Follow: There should never be two devices terminated on the internal SCSI chain. The SCSI card counts as a device, so terminating both the internal hard drive and the internal CD-ROM drive is redundant and will end up causing errors. The same applies to the external SCSI chain. A maximum of one terminator or terminated device should exist on the device physically at the end of the SCSI cable chain. Never use two termination devices.

The exception to this rule is the Mac IIfx, which needs a black terminator. The reason this terminator is necessary is that the IIfx is the only model that Apple made that doesn’t have a self-terminating SCSI controller. Even when no external devices are attached to the IIfx, the black terminator must be in place to terminate the internal devices. Additionally, if external devices are attached, this black terminator needs to be moved to the last device, or in some cases must stay connected to the case of the IIfx and “passed through” to other devices.

Active Termination and Passive Termination

Instead of using active/passive and all the variants, let’s deal with physical and electrical. A physical terminator consists of a small block you attach to the “out” connector on the back of a SCSI device. This terminator has a resistor that provides a voltage drop on two pins of the 25 or 50 pin cable. An electrical terminator is the method built into most SCSI hard drives now, and instead of using true resistors to provide the voltage drop, these devices use a reverse electrical current to control or “bleed” the voltage to the proper level. These devices are often called smart termination devices, but as is typical, they’re usually very stupid.

Let’s take the previous example, and mark the “smart” terminated devices with a “Q”.

(T)(Q)Hard drive <=> (Q)CD-ROM <=> (Q)Iomega <=> SCSI Card <=> (Q)SyQuest <=> Scanner(T)

In the case of the internal portion of this drive chain, I would disable termination on the CD-ROM and Iomega, so that the Hard Drive performs it’s own termination at full power. If not, then all three internal devices will battle out which reverse voltage termination circuit is compensating the right amount and make the chain unstable.

On the external portion, I choose to turn off active termination on the SyQuest and install a true terminator on the scanner. Different from above, I need to keep the scanner on constantly to assure a stable chain, but if I switched the SyQuest and the scanner, I would still opt to use my true terminator and keep the SyQuest termination off.

Termination and FireWire

SCSI II and SCSI III follow this same convention, but with FireWire the terminology – but not the actual circuitry – changed. A self-terminating device in FireWire is a 4-connector device, instead of a 6-connector device. FireWire uses 4 pins for data, 2 for an elusive concept called “power.” If a device, such as a Sony DVD camera, provides it’s own “power”, then only the 4-connector wire is necessary for data transfer. Of course, in this case, this is the only device that can be connected to a FireWire jack.

There is an exception to this. If I wanted to connect two Sony DVD cameras to my FireWire port, I’d need to purchase a “power adapter.” This adapter is a 6-pin to 4-pin to 6-pin adapter, all owning a”power connection” to the FireWire card, a jump through to the 4-pin Sony Camera, and a pass through to another FireWire device (in this case another Sony Camera). After placing one on the first camera, I need a second one to go to the second camera, as it’s not recommended that I directly plug my 6-wire adapter directly to the next device.

A bit of history and explanation is in order.

When SCSI was first released, the term “termination” became the hot topic. Termination is short for a concept called “termination power,” which provides a minimal voltage used for the detection of SCSI devices on a chain. On a SCSI device, installing resistors creates a managed short between a set of wires and shortcuts the circuit so that “termination power” does not pass beyond that device to another device. In this way, the circuit is completed, and signals can travel in a loop back to the controller card. When termination isn’t present on a device, the loop is extended to the next device, instead of being completed on this device.

In the past few years, hard drive manufacturers have been properly marking their drives with this “termination power” instead of just writing “termination.” The concept that a detection signal is going through the lines, instead of just a pure voltage that means nothing but heat dissipation is important, as termination power has become more important for device information in the SCSI II and SCSI III devices.

But in FireWire, the word termination has been left off the term, and now the concept of “device power” or just “power” is the norm. I find this rather confusing to AV and DV people unfamiliar with SCSI devices, because the respect and understanding isn’t present. Inside each “power adapter” for FireWire is a small electronic switch that tells the FireWire card whether it is the last device on the chain or not, by using these two “extra” wires. In the case of the first Sony camera, the power adapter completes the “termination circuit” for the Sony, detects the next power adapter in the chain, and does not provide termination resistance because it expect the next device to make the decision. In that way, the FireWire card now knows that there is a Sony Camera attached, and that another device might report in. When the second power adapter is polled by the card, it also reports a “self powered” Sony Camera, and tells the FireWire card that it (the power adapter) completes the power circuit (thus terminating it), because it does not detect another power adapter or 6-pin FireWire device beyond it. While the industry calls these power adapters, they’re simply FireWire terminators that override the “built-in” termination of these 4-pin devices. (You have to admit, by not including two wires, that’s a heck of a way to build in termination.)

Of course, a more expensive (or “intelligent”) power adapter could detect both 4-pin cameras being connected without the need of a second power adapter, but we’re awfully close to the kinds of problems that ethernet networks have (because we’re making a kind of hub), or the kinds of problems we have mixing active and passive terminators on a SCSI chain. The point is not to be confused or alarmed when the terminology switches in FireWire – the concept is very much the same as in previous SCSI modes.

(Final Note: FireWire is part of the SCSI-3 category, check the IEEE 1394 descriptions for more information).

Other Resources

Scott L. Barber posted this to Quadlist on December 12, 1998. It is reprinted with his permission.

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