passive terminators, which are what's built into most drives, are just a pair of resistors for each signal line, one to vcc and one to ground. this scheme is not ideal, but usually works though it does not provide the correct impedance. there are also capacitors across vcc and ground as there are across chips on a circuit board to filter the vcc and provide high peak currents when signals change state, they are not a part of the termination in the first approximation though obviously in the real world they do have a good and desired effect as they reduce the effects of inductance in the path from the regulator and ground to the terminator.
active terminators have a low voltage power supply set at the mid voltage between a high and a low signal on a line, and on each signal line a resistor of the correct impedance (that of the standard scsi cable) in series with that voltage source. this is ideal theoretically. forced perfect goes a step further and also actively clamps any signal ringing outside of the specified voltage range. i.e. it clamps voltages that are too high or too low on the signal line. this usually isn't necessary, but it can help at very high data rates with long cables which may not always have exactly the correct impedance and where there are many drives connected which create a "Y" in the signal line which will always create echoes of the signal. many of the drive makers have excellent tutorials on termination. quantum had a very good acrobat file, i don't know if it's still on the maxtor site or not (maxtor bought quantum but has continued to provide data on quantum drives, even the old ones the last time i checked so they may still have it). Joshua Coombs wrote: ------- > Internal terminators on drives are usually just resistor strips, > which work OK at lower data rates and bus loads. Active terminators > as the name implies, do more than use fixed resistance to control > signal bounce. They adjust on a line by line basis the termination > to maintain a uniform bus and clean signals. If you're running > external SCSI, I highly recomend external, Active terminators unless > you know the internal setup is active. ------- -- President George W. Bush, Vice President Richard B. Cheney, Secretary of Defense Donald H. Rumsfeld, and Attorney General John D. Ashcroft have committed violations and subversions of the Constitution of the United States of America. <www.VoteToImpeach.org> They should be charged with high treason and imprisoned frankly. If there is no rule of law there can be no civilization. -- G-List is sponsored by <http://lowendmac.com/> and... Small Dog Electronics http://www.smalldog.com | Refurbished Drives | -- We have Apple Refurbished Monitors in stock! | & CDRWs on Sale! | Support Low End Mac <http://lowendmac.com/lists/support.html> G-List list info: <http://lowendmac.com/lists/g-list.shtml> --> AOL users, remove "mailto:"; Send list messages to: <mailto:[EMAIL PROTECTED]> To unsubscribe, email: <mailto:[EMAIL PROTECTED]> For digest mode, email: <mailto:[EMAIL PROTECTED]> Subscription questions: <mailto:[EMAIL PROTECTED]> Archive: <http://www.mail-archive.com/g-list%40mail.maclaunch.com/> Using a Mac? Free email & more at Applelinks! http://www.applelinks.com
