Date: Sun, 16 Aug 98 17:32:00 -0000 From: mikebw@bilow.bilow.uu.ids.net (Mike Bilow) To: aic7xxx@FreeBSD.ORG Subject: AHA2790UW has speed-limit problems ? Message-ID: <5d708c02@bilow.bilow.uu.ids.net>
next in thread | raw e-mail | index | archive | help
Ross Harvey wrote in a message to Mike Bilow: RH> I would advise against doing this! Three reasons: I didn't advise in favor of overterminating the bus, either. :) RH> 1. The terminators have a DC load. The load from the first RH> pair of terminators is already at the specified max for the RH> drivers. This is true to a point, but the practical situation is that you have a very large safety margin that is nearly an order of magnitude. In terms of DC load, an extra terminator will appear in parallel with the existing two. RH> 2. At the point where the incident wave hits the middle RH> `terminator', the bus Z becomes Z/2. A terminator of resistance RH> R is equivalent to an infinite length transmission line of RH> impedance Z==R. The middle `terminator' then appears as a fork, RH> with an infinite length line one way and the remainder of your RH> SCSI `bus' (it's not really a bus any more!) going the other RH> way. For an incident wave with voltage step V, you get an RH> inverted reflection of V/2 heading back to the src and a RH> truncated forward wave of V/2 heading towards the original dst. RH> At the source, the driver will put out another V/2, (if it can, RH> it's now being asked to sink or source more mA than its data RH> sheet allows) and this will reflect forward and back as V/4 at RH> the middle `terminator'. I don't mean this to sound facetious, and I'm not even disagreeing with your analysis, but what you have constructed is a theoretical argument against attaching peripherals to the SCSI bus. Admittedly, you have to introduce impedance discontinuities into the system at numerous places, because a SCSI bus without peripherals may not be especially useful. It is certainly possible to push this too far, as you say, but the issue is really how much is too far? I have never seen any real SCSI bus fail due to overtermination even when the bus was absolutely packed full of peripherals, and the loading resulting from the addition of a third terminator is roughly comparable to what you get by adding a couple of peripheral devices. It is precisely this problem of device loading which causes the design problem in the first place. SCSI devices must be designed to support anywhere from two to eight device loads, and this represents a significant range. It is a common mistake to say that these loads are high-impedance devices in contrast to the terminators and therefore do not affect the system impedance, but everyone knows from practical experience that this is not true. In fact, the average SCSI bus is probably operating underterminated because it had to be designed to accommodate such a wide range of cable lengths and peripheral counts, and many problems are the result of this undertermination. Sure, it is possible to make all kinds of clever circuits which sense impedance and correct for the ideal design condition, and forced perfect termination is a good example of such a system. However, economic considerations usually compel device designers to compromise on this, and to design using devices that are simply fixed with sufficient bias to operate linearly over an acceptably wide range of conditions. It usually works, but this sort of engineering falls flat on its face when you start using long cable lengths, adding lots of devices, and increasing data rates. Ultimately, where you and I disagree is your assertion that overtermination _always_ results in increased noise (or, to think of it another way, decreased signal-to-noise ratio). This can certainly happen, but it is not always true. RH> You might observe this to work, anyway, in some specific RH> example case. This might be due to a combination of slow RH> drivers and a short bus, so that it is partly a lumped load and RH> not a transmission line, and because the drivers fortunately RH> had extra capacity, they didn't burn out, and you didn't notice RH> the possibly shorter drive lifetimes. Or, perhaps, because an RH> end terminator was defective or not really enabled as expected. RH> But see #3. I readily concede that more sophisticated techniques, particularly forced perfect termination, are preferable to deliberately overterminating the bus. And, while your theoretical arguments have some merit, I can tell you from long experience that I have never blown up a single device due to overtermination, and I have blown up many through other causes! I have watched enough of these things on analyzers and oscilloscopes to know that nearly all devices are necessarily designed so that they tend to be close to the low side of their range in actual operation. For that matter, I have on a few occasions tested a SCSI bus and learned just how much safety margin really exists, shortly thereafter using a fire extinguisher on the SCSI cables. As for overtermination working only when the system operates as a lumped load rather than a transmission line, it is worth pointing out that the same technique has been used successfully in real radio antennas for decades, and that several classical designs (such as the rhombic) actually depend upon it. RH> 3. The correct solution is to recycle the SE HW and go RH> differential, LVD, or FC. Even if you can tweak it into working RH> by upgrading terminators or cables or by --oh dear-- *refolding RH> and untwisting the cables*, whatever, you can bet money that RH> you have no noise margin. I agree, but your "correct solution" involves replacing all of the hardware! -- Mike To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe aic7xxx" in the body of the message
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?5d708c02>