This tech note helps you identifying odd SCSI errors, including their potential location and causes in your tape library so you can fix the problem promptly. It also includes configuration tips so your tape library runs at its best, as well as a glossary of terms.

We encourage you to download this tech note, Identifying odd SCSI errors, in PDF format for future reference and use.


Error codes (ec=) such as “unrecoverable data error” or “failed to write data to tape” are often caused by dirty tape heads, or a defective tape / section of a tape. If several types of errors (e.g. both “media errors” and “SCSI port errors”) occur together or there is doubt about the exact location of the error, more than one problem could be occurring. In that case, every aspect of the SCSI bus needs to be checked.


  • Tapes: Tape drive manufacturers specify which brands and lengths are compatible with their data storage equipment. You should always check your tape drive’s documentation for full compatibility, since unapproved tape brand or type can consistently fail. It is also important to only use tapes of the proper length since some tape drives can use various lengths but others can’t. Tapes can vary in thickness, tension, and particle density and tape drives are designed to properly handle those differences without causing damage to themselves or the tapes.
  • Cleaning cartridges: You should regularly clean your tape drives by using cleaning cartridges, which not only clean the tape drive head, but also the full tape path. Cleaning will prevent both read/write and tape positioning problems. Use only the manufacturer’s recommended cleaning tapes and follow the instructions. Some cleaning cartridges are only good for a certain number of uses. If used more than specified, they will reach their end and not perform any cleaning. Most manufacturers recommend cleaning the tape drives every 25 to 30 hours of use, as well as after using a new tape for the first time.
  • Tape drives: Just because a tape drive is new doesn’t mean its heads are clean, so while head cleaning on a new drive is not necessary, it also can’t hurt. If a tape drive was not cleaned according to the manufacturer’s recommendations, there could be build-up on the heads and a cleaning tape won’t be able to remove it. Such build-up, or a drive hardware failure, can cause media errors. If the same errors occur after trying new tapes and cleaning the drive, it needs servicing. You could also try performing the same backup using a different software program.


The SCSI bus consists of devices and cables that create electrical connections throughout the length of the bus. Problems can arise at several different levels:

  • Physical: Every device must have a connection for both power and data, so check for bent pins on the SCSI cable connector. Check the SCSI cable for obvious damage, kinks, or pinches. Look at the internal cables and make sure extra cable is not wound up into a coil, or bunched up right next to the power supply. For external devices, try swapping the ports that the cable and terminator are plugged into.
  • Electrical: You should measure the voltage at the power connector of a problem device. Voltages on the peripherals should both be within +/- 5% (4.75VDC – 5.25VDC and 11.40VDC – 12.60VDC). Voltage is usually not a concern for internal devices unless a desktop unit with an underrated power supply is used as a server. External devices have their own power supply and may temporarily lose enough power to cause a “Unit Attention” error. Proper flow of the data is dependent on every aspect of the bus and can only be properly measured with a SCSI analyzer.
  • SCSI IDs: SCSI IDs 0 & 1 should be used only for hard drives unless the adapter’s documentation advises otherwise. Tape devices should have lower IDs than the other non-hard drive devices on the same bus. A tape changer that uses multiple IDs should have a lower ID than the tape drives installed in it. If a device is set to the same SCSI ID as the adapter or another device, this will often cause “phantom” devices to be seen at IDs when there are no such devices present.
  • Bus length: Improper bus length can cause SCSI problems when external devices are connected. Therefore you should aim to keep the SCSI bus length as short as possible. When calculating total bus length, you need to add stub length, which is the distance from the external port to the device inside, usually around 3 inches.
  • Termination: Proper termination is key to the reliable operation of the system and the SCSI bus so you should watch for these potential problems:
    • An SCSI bus with only one terminator (described as under-terminated) will generally perform slowly and suffer parity errors.
    • An SCSI bus with too many terminators (described as over-terminated) will perform better than a bus with only one terminator, but the transfer speed will still be slow.
    • An SCSI bus with no terminators will probably not function at all.
    • Improper termination can cause many problems, such as a computer appearing to run slowly or reporting errors. Several peripherals tend to be hard to terminate, either because the host is extremely fast and intolerant of low-quality cables and terminators, or the device wasn’t originally designed for SCSI use and a poorly designed SCSI interface was grafted on as an afterthought (e.g. scanners).


Device map:

  • Creating a complete device map of your system is very helpful. Start with the slots in the computer, recording each slot number and the adapter installed in it. Next, show the adapter model, I/O Port Address, Memory Address, DMA, IRQ, whether it is terminated, and each device attached to it. For each device, show the type (hard drive, tape drive, CD) make, model, firmware, SCSI ID, SCSI I, II, III, narrow or wide, terminated or not, and its physical position in the chain. Finally, record cable type, length, any special adapters or connectors, and terminator types.
  • Looking at the Device Management view in the ARCserve manager can help you create a device map. All devices will be listed on the left side, starting with the SCSI adapter. The devices attached to it will show up next. You can highlight a device to see its details on the right side. Tape drives have a Summary and a Detail tab on the right side. Other devices will only have a Summary tab available. For SCSI adapters, the name of the driver will be listed instead of the model number. Note the adapter and each device attached to it. For SCSI devices, make a note of the Vendor, Product Name (usually the model #), firmware, and the SCSI ID, and SCSI I, II, III. The Tape log can record hardware configuration if you set “Message Level” to “Detail”, and “Device to be Monitored” to “All Devices”. Stop and start the tape drive and turn off the tape log when you are finished.


  • Sub-standard or unshielded external cables can cause SCSI problems. SCSI cables should be noticeably thicker than printer cables and their ideal impedance (resistance) is 11 Ohms. The impedance of lower-cost cables is often as low as 60 to 80 ohms. Wires in the middle of the cable can also have lower impedance than the outer ones if not individually shielded.
  • It is better to use an internal cable with the same number of connectors as the number of attached drives. If a single drive is connected to a “three-connector” cable, you should attach the device to the last connector and leave the empty connectors in the middle. Cables should match up from one device to the other evenly. Special cables or adapters to connect a wide device to a narrow one shouldn’t be used. If the adapter port is wide, only connect wide devices to it, and vice versa for narrow devices.
  • Don’t mix and match flat and round cables on either side of the adapter (using flat cable internally and round cable externally is OK). By mixing cable types, the devices, even those with active termination, will have significant difficulties managing proper impedance.
  • Try keeping a minimum of one foot of cable length between each device. Sometimes sub-assemblies use very short cables to connect internal devices.
  • Cable length between devices should not exceed 3ft. Don’t leave any SCSI cables not connected to a device, even if they are terminated. Compaq internal cables are an exception.


The SCSI bus must be terminated on both ends. When all SCSI devices are external, the host adapter provides termination on one end, while an external terminator is attached to the last device on the external chain. If there is a mixture of internal and external devices, the host adapter is placed in the middle of the physical chain and must not be terminated. You should use the same type of terminators on both bus ends. Three types of termination can be used on or after the last device on the SCSI bus:

  •  Passive termination, which consists of resistors only.
  • FPT (Forced Perfect Termination), which uses diode clamps to eliminate overshoot and undershoot.
  • Active termination (only for SCSI-II & III), which uses a voltage regulator to ensure the SCSI signals are always terminated to the correct voltage level. Since active termination is 110 ohms, it is better matched to the bus than passive termination is, and it can operate faster without SCSI retries. With active termination, you need to enable Termination Power on all devices.


  • Remember to always upgrade hard drives and tape drives to the latest firmware release. However, don’t assume that because a drive was certified with the operating system’s previous version, it is automatically certified with the newest one.
  • Power up and fully initialize external devices prior to starting the server.
  • Put scanners at the end of the SCSI chain.
  • Connect slower devices closer to the adapter.
  • Don’t leave any unpowered devices connected.
  • Most adapters do supply Term. Power so verify your adapter does (e.g. the Adaptec AHA1510 doesn’t). If the adapter supplies Term. Power, check if the SCSI devices have settings for “Termination Power” or “Unit Attention”. If they do, set Term. Power to come from the host adapter, or pin 26 on the SCSI Bus.


  •  Don’t use more than two PIO adapters in the system (e.g. AHA1510 or AHA1520), or more than four ISA adapters (e.g. AHA 1540 or AHA1740 in standard mode). Bus Master adapters are limited only by the number of bus master slots (such as AHA1740 in enhanced mode, AHA2740, AHA2840, or AHA2940).
  • For bus mastering, reduce the bus “on time” when multiple adapters are being used. Verify the slots are bus master, since not all EISA, VESA Local Bus, and PCI slots are.
  • If an ISA (or EISA) adapter and a PCI adapter are installed in the same system, check the system CMOS supports allocating separate IRQs to the ISA bus and the PCI bus. In the CMOS, mark the IRQs used by ISA/EISA boards as “Used” so that the BIOS will not try to assign the IRQs to other boards.
  • In PCI systems, make sure the mainboard BIOS is current. Some PCI adapters have special manufacturer recommendations, such as setting Interrupt Type or Line to “A”, and Triggering Interrupt to “Level”. The BIOS on the adapter should be enabled only if the adapter is connected to the boot drive and if the BIOS is disabled on the other adapters.
  • Unsupported devices can cause SCSI problems, so verify with the adapter manufacturer that all attached devices are certified. Sometimes you can configure the adapter for Disable Sync Neg., and for the minimum transfer speed.


The SCSI Disk Utilities can be used to scan the bus and report attached devices, their IDs and device info.
– Enter SCSI Select by pressing CTRL-A
– Configure View Host Adapter Settings (different versions of BIOS may have unique features)
– Options are Parity Checking, Termination & Options for SCSI Device Config, Advance Config
–Parity Check: should always be enabled
–Termination: can be left to automatic (2940s have auto detect for termination; leave it at automatic)
– Advanced
– PnP Plug & Play SCAM support SCSI configured automatically: disable
– BIOS enabled if has boot drive otherwise disable
– Support Removable – boot only – enabled by default; used only for Bernoulli/SyQuest drives
– Extended BIOS translation enabled by default; only needed for hds > 1gb under DOS: disable
– Display setup message during startup; with disabled ctr A still works
– LUN support (disabled by default): must be enabled for Changers that share an ID
– BIOS support for more than two drives; only applicable to early DOS
– BIOS support for removable CD-ROM
– Reset SCSI Bus is enabled (only on older 2940): disable
– Support for Ultra SCSI

SCSI Device Configuration Option 
(Shows the options in column on left and SCSI ID in rows)
– Init Sync Neg (enabled by default): disable; determination is then done by device, not adapter
– Max Sync Xfer rate:default whatever its maximum is; set to min. for all 1/4″ & DAT Tape Drives
– Unit Wide Negotiation: should be enabled only for Wide devices
– Enabled Disconnect: should be enabled
– Send Start Unit Command: disabled

If an option is not needed, it is best to disable it.


ACTIVE TERMINATION: An active terminator has one or more voltage regulators to produce the termination voltage rather than using resistor voltage dividers. SCSI-II uses a voltage regulator to ensure the SCSI signals are always terminated to the correct voltage level. Active termination consists of individual 110 W resistors powered by a regulated +2.85V supply on each of the signal lines. There is no longer any direct connection to ground in the terminator, and the voltage must be more stable than passive termination. If the power comes from the TRMPWR line on the bus, it’s regulated from +5V to +2.85V by the terminator itself.

ASYNCHRONOUS DATA TRANSFER: A method of SCSI data transfer, originally introduced with SCSI-I. The asynchronous protocol uses a traditional REQ/ACK handshake.

BIOS: An acronym for Basic Input/Output System. With a SCSI host adapter, the BIOS is used to control SCSI hard disk drives and perform the boot function. The BIOS can also contain useful software utilities, which can be used to change the host adapter settings, format disks, and run simple SCSI diagnostics.

BOT (Beginning of Tape): Physical beginning of the tape.

BUS MASTERING: A high performance method of data transfer, in which the host adapter’s on-board processor handles the transfer of data directly to and from a computer’s memory without intervention from the computer’s microprocessor.

CACHING: A process where data requested by the computer’s operating system is retrieved from RAM instead of from a hard disk (or some other mass storage media). Cache can be on the host adapter, on the motherboard (controlled by the operating system), or on the SCSI device. Operating system vendors such as Novell and Microsoft recommend cache not be used on the host adapter, since today’s operating systems can’t work in conjunction with host adapters with on-board RAM. This leads to degradation in performance, and possibly data loss.

CAM (Common Access Method): A programming standard encapsulating the SCSI functions into a standardized calling interface.

CCS (Common Command Set): A set of commands specified as part of the SCSI-I specification and expanded in SCSI-II. CCS makes it easier to write drivers for SCSI devices because you can rely on specific functionalities available for each class of device (e.g., hard drive, tape drive, etc).

COMMAND QUEUING: In SCSI-I, initiators were limited to one command per LUN (e.g. a disk drive). Now up to 256 commands can be outstanding to one LUN. The target is allowed to re-sequence the order of command execution to optimize seek motions. Queued commands require tag messages, which follow the identifier.

COMMAND SWAPPING: A SCSI host adapter feature allowing the host adapter to support up to 255 simultaneous commands.

DEVICE DRIVER: A software program enabling a PC to communicate with peripheral devices such as fixed disk drives and CD-ROM drives. Each device requires a different driver, and device driver programs are stored on a PC’s fixed disk and loaded into memory at boot time.

DIFFERENTIAL: A term referring to the electrical characteristics of the signals used on the SCSI bus interface. Differential signals occupy two conductors with a positive (+) and negative (-) polarity component of the signal. This minimizes the effect of common mode signal noise and allows the SCSI bus to operate reliably over greater distances at a higher speed. Any noise will affect both lines in the same direction, so the voltage difference remains the same.

(DMA) DIRECT MEMORY ACCESS: A mechanism allowing hardware control of the transfer of streams of data to or from the main memory of a computing system. This mechanism may require host software setup. After initialization, it automatically sequences the required data transfer and provides the necessary address information.

EOD (End Of Data): Location on the tape where the last session stopped.

EOM or EOT( End Of Media or Tape): Physical end of the tape.

EXTERNAL TERMINATION: Normally, external terminators are separate devices that attach to the last peripheral on the bus. End-cap terminators have a single 50-pin Centronics male connector and attach to the open connector on the last device on the bus. Other types of connectors can be used on external drive cables and external terminators, but 50-pin Centronics are the most common.

FPT (Forced Perfect Termination): Uses diode clamps to eliminate overshoot and undershoot.

h: Stands for hex, or hexadecimal, a counting system commonly used in computer science based on 16 instead of 10 (decimal). An ‘h’ after a number indicates this is a hex number.

HVD (High Voltage Differential): HVD transceivers are high powered and can’t be integrated into a controller chip, requiring separate external transceivers.


INTERNAL TERMINATION: Refers to termination built into the SCSI interface of a host or peripheral. The most common form is SIPP (Single In-line Pin Package) resistor network. Another type is surface-mount chip terminators soldered onto the printed circuit board (PCB) of the SCSI device and enabled/disabled by a jumper or switch.

INTERRUPT 13: Software interrupt for disk I/O used by DOS. DOS does ‘Interrupt 13 calls’ to read or write from a diskette. A SCSI host adapter translates these Interrupt 13 commands into SCSI commands for SCSI disk drives.

INTERRUPT 19: Software interrupt that handles the boot function. The boot code is usually handled by the motherboard BIOS, but can optionally be handled by the host adapter BIOS.

IRQ (Interrupt Request Channel): The IRQ of a host adapter can have different settings by changing jumpers and/or switch settings on the adapter board.

LVD (Low Voltage Differential): Ultra2 SCSI uses a low-voltage differential interface. It is a physical interface with power low enough to allow integration within the SCSI controller chip, and a differential interface with a fail-safe biasing built into the receivers.

LUN (Logical Unit Number): Used by some changers sharing a SCS-I ID with the tape drive. For example, the changer could be on SCSI ID 3 Lun 0 and the tape drive SCSI ID 3 LUN 1. Changer commands will be directed to ID 3 LUN 0, and tape drive commands to ID 3 LUN 1.

MULTI-THREADING: When a host adapter has more than one outstanding command to two or more SCSI devices.

NARROW SCSI: Term attributed to 8-bit standard SCSI devices.

PASSIVE TERMINATION: Pairs of terminating resistors attached to individual SCSI signal lines, a positive supply voltage of +5 VDC and a reference to ground. Both resistors in each pair have different values. One resistor is 220 W, which is tied to +5V. The other is 330 W, which is tied to the ground of the +5V. They come without an LED on them for terminator power.

PLUG AND PLAY (PnP): Adapters designed to the Plug and Play standard will self-configure, and automatically resolve system resources such as interrupts (IRQ), DMA, port addresses, and BIOS addresses.

PORT I/O ADDRESS: A window through which software programs communicate commands to an installed host adapter board.

PROGRAMMED INPUT/OUTPUT (PIO): A method of data transfer in which the host microprocessor transfers data to and from memory via the computer’s I/O ports.

SCAM: (SCSI Configures Auto Magically): Also known as Plug and Play for SCSI. Using this specification, the SCSI host adapter can automatically select SCSI IDs for itself and other attached SCSI devices. It can also enable/disable termination as required to properly terminate the SCSI bus.

SCSI (Small Computer Systems Interface): A PC bus interface standard that defines standard physical and electrical connections for devices. SCSI provides a standard interface that enables different kinds of devices, such as disk drives, magneto optical disks, CD-ROM drives, and tape drives to interface with the host computer.

SCSI DEVICE: A device such as a host adapter board, hard drive or CD-ROM drive that conforms to the SCSI interface standard and is attached to a SCSI bus cable. The device may be an initiator, a target, or capable of both types of operation.

SCSI DISCONNECTION: Occurs when a device will release control of the bus while still processing a command. The device will automatically reconnect to the bus later when it has either completed the command or needs more data.

SCSI OVERHEAD: The time it takes for the host adapter to internally process a SCSI command. RISC based host adapters have the advantage of extremely low SCSI overhead, which greatly increases overall system performance.

SE Single-ended SCSI (normal SCSI): Single-ended SCSI uses one line for each signal and all lines using a common ground reference, which enables you to use less expensive hardware. One disadvantage is its vulnerability to noise, because all lines share the same ground reference. The SE spec calls for very tight termination tolerances, but the passive 132-ohm termination is mismatched with the cable impedance (typically below 100 ohms), which can be a problem at higher speeds.

SOFT WRITE OR READ ERROR: When the tape drive tries to write or read the tape and fails. The tape drive will advance the tape a little and retry the operation. This process will continue until the retry threshold is met and the error code Media or UDE occurs.

STUB: A short section of interconnecting cable from the main SCSI cable to the external device. Usually around 3 inches long, the spec calls for it to be less than 4 inches.

SYNCHRONOUS TRANSFER: A method of SCSI data transfer where the SCSI host adapter and the SCSI device agree to a transfer rate that both support (known as synchronous negotiation). The traditional REQ/ACK handshake is not done for each transfer, making it faster than asynchronous.

TAGGED QUEUING: A SCSI-2 feature that increases performance on SCSI disk drives. With tagged queuing, the host adapter, the host adapter driver, and the hard disk drive work together to increase performance by reordering the requests from the host adapter to minimize head switching and seeking.

TARGET: The SCSI device attached to the adapter.

TERMINATION: A device that attaches to both ends of an electrical bus and prevents reflection or echoes of signals that reach the end of the bus. It also makes sure the impedance is correct.

TERMINATION POWER: Usually present on the bus at a pin (wire) reserved for the purpose of distributing power to be used by external termination devices. This pin is called “Term Power” (or the more abbreviated form TRMPWR) and can be supplied by any or all devices attached to the bus (host or peripheral).

UNRECOVERABLE DATA ERROR (UDE): When the tape drive is unable to successfully write or read to tape. This error will cause the current operation to cancel.


  •  SCSI-1: The origin SCSI command set, which defined the SCSI transfer rate to be 5 mb/s.
  • SCSI-2: Extends the original CCS specification to include support for new devices.
  • FAST SCSI-2: Uses the SCSI-2 command set and the transfer rate is increased to 10 mb/s.
  • FAST SCSI: The term “FAST” is generally applied to a SCSI device that can do synchronous transfers at speeds in excess of 5 mb/s.
  • Ultra SCSI: Enables very fast data transfer rate on the SCSI bus, up to 20 mb/s (40 mb/s for Wide SCSI host adapters).
  • Wide SCSI: Includes additional SCSI commands to the SCSI-2 command set and the transfer rate is increased to 20mb/s.
  • SCSI-3: This specification addresses some of SCSI-2’s limitations. It includes support for optical fiber, longer cables, and more than eight targets per bus and runs at 20 mb/s.
  • Serial SCSI: Involves using fiber-optic or high-speed copper (IEEE P1394, Fiber Channel, and SSA – Serial Storage Architecture) with speeds ranging from 51 mb/s to 1 Gb/s.