Need termination on each end of the scsi chain. Typically, the motherboard supplies it on that end. Need to enable it on the last device on the other end.
From <http://www.scsita.org/aboutscsi/SCSI_Termination_Tutorial.html> Why terminate? A pulse propagating along the SCSI bus will 'reflect' from any part of the bus that is different from the rest of it. These reflections add and subtract in odd combinations and cause the original pulse to be distorted and corrupted, thereby causing data loss. To prevent or minimize reflections from the ends of the bus, terminators are added to "absorb" the energy from the pulses. The terminators on the SCSI bus hold the bus in a negated state, any one of up to 16 drivers could be driving the bus or none. The bus is held in the negated state as required by the SCSI protocol. The original drivers on a SCSI bus only asserted (Open collector) and the terminators were used to negate the bus. Drivers that assert and negate the bus are known as active negation drivers. The negation is less current than the assertion, the drivers are designed for 2 terminators negating the bus. Where do you terminate the bus? [Terminate: from the latin terminus meaning "the end".] Termination must be present at two and only two positions on the SCSI bus, at the beginning of the bus, and at the end, and must occur within 4 inches of the physical ends of the SCSI bus. Often, the host will be installed at one end of the bus, and will provide one of the two terminations required. When do you terminate the bus? Always. Period. Whenever you want the SCSI bus to operate reliably! How are terminators powered? Terminators are powered from the "term power" line on the SCSI bus. Term power can come from any device on the bus, and is provided by either the host, a drive on the bus, the backplane, or any combination thereof. Term power is provide through a diode and fuse - historically - the drop accross the diode and cable allows for a term power range of 4.0 to 5.25 volts. What kinds of terminators are available? Internal: some SCSI drives (pre-LVD) had passive terminators installed in the drive, and could be enabled/disabled by setting a jumper or inserted/removed from a socket. Drives with LVD interface do not have internal termination due to reduce bus loading, and the fact that in a multi-drive environment, only one drive needs to be terminated. External: In-line (also called 'feed-through') terminators connect in series with a SCSI device. Close-ended terminators plug into a bus connector or may be crimped on the cable. Passive: resistor networks driven directly by the term power on the bus. Active: resistor networks driven by voltage regulators inside the terminator. All LVD terminators can be assumed to be 'active'. What is the difference between passive and active termination? Passive termination is a resistor network, 330 ohms from ground to the signal line and 220 ohms from the signal line to termpower, which is an equivalent impedance of around 132 ohms. Since typical characteristic impedance of most cable is between 85 and 110 ohms, this is not a great match. The negation current varies with the term power voltage which can be from 4.0 to 5.25 volts, the pull up current would range from 16.5 to 24 mA. This would often required multiple steps to insure the voltage reached the 2.0 volt at the receiver. An active termination incorporates a small voltage regulator which generates a stable termination voltage from the raw Term Power (which may fluctuate) and allows the termination resistance to be lowered to 110 ohms, more closely matching the cable impedance. Active termination is recommended for all Single Ended buses at any speed, and is required for Single Ended Ultra SCSI speeds and above. New systems with active terminates may be powered from a 3.3 volt source, termpower can be from 2.7 to 5.25 volts. Other important terminology related to termination, for the discussion that follows: Active Negation. Asserting a signal on a Single Ended SCSI bus requires driving the signal low; de-asserting (negation) is the opposite, allowing the signal to be pulled high by the terminator. For fastest bus speeds, the signal may be 'driven' high by a driver in either the host or SCSI device, thus 'active negation'. Active terminators must not prevent active negation. Active negation is required for Ultra SCSI (Fast-20). High Byte. The upper data byte (bits 8-15) on a wide bus. This includes eight data and one parity bit, for a total of nine bits. Low Byte. The lower data byte (bits 0-7) on the SCSI bus. This includes eight data and one parity bit, for a total of nine bits. Multidrop. A SCSI bus with more than two devices on it. Multimode LVD/MSE. LVD devices respond to the voltage on the DIFFSENS line on the bus, and can switch modes from LVD to Single Ended. Active terminators must drive the diffsense line with 1.3 volts with low current, and therefore be able to respond to the DIFFSENS line, such that if the voltage is < 0.7v, the terminator will be in the Single Ended mode; > 0.9 and < 1.9v is the LVD mode. Should the DIFFSENS line see > 2.4v, the terminator should tri-state off the bus as it is seeing HVD (High Voltage Differential). Point-to-point. A SCSI bus with only two devices on it; one at each end. Stub length. The distance from the drivers/receivers on the SCSI device to the bus where it is plugged in. This may include the traces on the drive pcb, inline terminator, etc. (back) Michael S. Macdonald wrote: > I concur with Jim, but always go the opposite way > ie: > moboSCSI BUS 0 <-- termination here only. > ---drive(ID1)---drive(ID5)---drive(ID3)---drive(ID2) > > Cheers...Michael > > >>SCSI voodoo. Make sure that each device has a different ID and ensure >>that the last *physical* device on the chain is the only one terminated. >> I had exactly the same problem and found that I had the termination on >>on two devices. >> >>For example... >> >>moboSCSI BUS 0---drive(ID1)---drive(ID5)---drive(ID3)---drive(ID2) <-- >>termination here only. >> >>Jim > > >>>On Fri, 26 Jul 2002, pamark wrote: >>> >>> >>> >>>>I need help. I have a power tower pro 225 with 384 megs of ram and os >>>>9.1 installed. I have one internal Matshita cd drive that came with the >>>>machine and one Yamaha 6416 cdrw that is also installed internally. I >>>>have one 2gb hard drive and 2 17gb hard drives all internal. I think >>>>that about covers my system. >>>>Next, I am having trouble getting my system to read cd's. 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