At 5:03 PM -0600 10/28/2002, Jeff Walther wrote: >Please forgive if any details are inaccurate, but this will get the >gist across. > >Termination Enable does indeed enable termination. Termination, at >it's simplest, is a resistor across each data line/Ground line pair. >This damps the signals on the data lines (wires) by dumping them into >Ground in a controlled manner. If you do this too early in your >SCSI chain, the signal propagates no further, and devices further >down the cable get no signals on the data lines. That's why >Termination should be at the ends of the cable chain and no where >else.
It has been a very long time since I took transmission theory but IIRC if you terminate in the middle of the line you still see some signal further down stream but I don't recall what the attenuation is. > >If you do not have Termination at the end of the cable chain, the >data signals can reflect at the end of the cable and propagate back >in the direction from which they came, causing interference with >following signals. So it is essential to have termination at both >ends of your SCSI cable, though sometimes SCSI will operate without >it. Overall a pretty good semi-technical description of termination. > >I'm a little hazy on exactly how termination power is used, but >basically it's a power supply line at a regulated voltage used for >termination. You should have only one device with Termination Power >enabled on your SCSI chain. In almost every case, the motherboard or >SCSI card supplies Termination Power, so you should not have it >enabled on any of your devices. The termination is via a resistor to ground and one to power. Thus it terminates the transmission line and floats the inputs at a proper voltage. To accomplish this a power source is needed hence termination power (+5V). This is known as Passive Termination. If you look at the resistor packs they will likely indicate a resistance of 220 ohms and 330 ohms. The R packs are wired with each signal pin connected to a 220 ohm and a 330 ohm resistor. The other end of all the 220 ohm resistors are connected to a pin and there is another pin connected to all the 330 ohm resistors. This provides a termination impedance of 132 ohms and a float voltage of either 2 or 3 volts depending on which way they are connected (I don't think I've ever seen it listed for SCSI). Active Termination uses a voltage regulator to supply the termination power and a resistor is connected from it to each signal line. The voltage regulator operates at the same float voltage of the resistor divider in a passive termination scheme. Depending on how it is down it draws less power than passive termination. Off hand I don't know of any other advantages of it. > >Having Termination Power enabled on multiple devices is one of the >most subtle and frustrating causes of SCSI chain problems. However, >some machines such as the Mac Plus and some of the PowerBooks do not >provide Term Power and so a device needs to supply it on those >machines. Also, some cheaply made SCSI cables do not contain the >wire which is meant to carry Termination Power, and on a SCSI chain >using such an inferior cable, you'd want to enable Term Power on one >device after the break which the cheap cable causes. Or get rid of a cheap cable before it bites you in the posterior again. -- Clark Martin Redwood City, CA, USA Macintosh / Internet Consulting [EMAIL PROTECTED] "I'm a designated driver on the Information Super Highway" -- PCI-PowerMacs is sponsored by <http://lowendmac.com/> and... Small Dog Electronics http://www.smalldog.com | Refurbished Drives | -- Sonnet & PowerLogix Upgrades - start at $169 | & CDRWs on Sale! | Support Low End Mac <http://lowendmac.com/lists/support.html> PCI-PowerMacs list info: <http://lowendmac.com/lists/pci-powermacs.shtml> --> AOL users, remove "mailto:"; Send list messages to: <mailto:pci-powermacs@;mail.maclaunch.com> To unsubscribe, email: <mailto:pci-powermacs-off@;mail.maclaunch.com> For digest mode, email: <mailto:pci-powermacs-digest@;mail.maclaunch.com> Subscription questions: <mailto:listmom@;lowendmac.com> Archive:<http://www.mail-archive.com/pci-powermacs%40mail.maclaunch.com/> Using a Mac? Free email & more at Applelinks! http://www.applelinks.com
