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Hot Swap

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What is Hot Swap?

The term "Hot Swap" refers to the common practice of either inserting, or removing SCSI disk drives in an operating bus, typically used in RAID subsystems or JBOD (just of a bunch of disks) environments. The ability to "Hot Swap" a disk drive is beneficial to customers. It allows them to remove potentially defective drives from the system, or upgrade capacity without having the inconvenience and expense of taking the entire system down to replace the drive. The ANSI documents cover this function under the chapter heading "Removal and Insertion of SCSI devices". Four distinct levels of functionality are defined in Table A.

The term "Hot Swap" is not actually defined in the ANSI standards, or the draft standards under development. It is interpreted as "the very restrictive Level 4 Removal and Insertion of disk drives." To avoid confusion, the two terms are linked together as "Level 4 Hot Swap."

The main difference between Level 4 and the easier levels is that the bus is allowed to operate (move data or operate in any legal SCSI bus phase). Since inserting a disk into any powered bus will result in some level of electrical transients, it is necessary to insure that those transients do not interfere with, or corrupt the control of data signals present on the bus.

Table A




Power Applied to Device

Bus State

Ground Connection to Drive

Device Circuitry Connected to Bus Pins


Cold Swap






Hot Swap while reset


Held in reset state

Must be made and maintained for 1 msec before, during, and after insertion/ removal*



Hot Swap while bus idle


Held idle (no ongoing I/O processes during insertion/removal

Same as Level 2

Must remain glitch-free during power up or power down


Hot Swap on an active bus


Bus may have active I/O processes ongoing, but device being inserted or removed must be idle

Same as Level 2

Same as Level 3

*Achieved on Quantum products using SCA-2 connector


Hot Swapping with Quantum’s Low Voltage Differential (LVD) Drives

Quantum's Atlas III and Viking II Low Voltage Differential (LVD) drives are designed for use in "Hot Swap" applications within a properly designed and configured SCSI system. With reasonable care, system integrators can design storage arr ays and/or SCSI buses using Quantum Atlas III and Viking II disk drives that will be "Hot Swap Level 4" tolerant.

Two metrics that are key to being "Hot Swap Level 4" tolerant:

  1. When a drive is being hot inserted onto an active bus, a smaller capacitance on the SCSI bus pins will create a smaller (less charge) transient spike on the bus.
  2. When a drive is the closest drive to the connector at which another drive is being hot inserted, it is desirable that the input receivers ignore (low pass filter) short low amplitude transients.

It is not possible for individual component suppliers (e.g. targets, initiators, backplanes, terminators) to absolutely guarantee that the system can operate under "Hot Swap Level 4" conditions. Therefore, the system integrator must accept some of the burden of assuring that the system can meet "Hot Swap Level 4" operational criteria.

System Integrators

LVD SCSI devices may require more stringent system design to tolerate transients that occur during Level 4 insertion or removal.

System integrators must consider the following principles when designing their backplane or bus:

  1. Larger connector-to-connector spacing causes the transients associated with "hot swapping" to be dramatically attenuated before reaching an operating drive.
  2. Larger SCSI signal-trace capacitances (lower impedances) are better for minimizing transient spikes, but can cause other bus problems unrelated to "Hot Swapping." Draft standard SCSI-3 SCSI SPI-2 (SCSI Parallel Interface), Rev 18, Sec. 6.6.2 shows the trade-offs between trace capacitance and drive to drive spacing.
  3. The universe of drives needs to be homogeneous impedance. A 15pF drive Hot Inserted into a rack of 15pF drives will be fine. But, a 30pF drive Hot Inserted into a rack of 10pF drives with very close connector spacing is more likely to cause problems.

Hot Swapping in Single-Ended vs. Differential Bus


Inserting a device onto an operating bus can be modeled as a capacitor (the bus pin of a disk drive) with zero volts across it being abruptly connected to the SCSI bus. It is required that the charge flowing from the bus into the discharged capacitor not interfere in any way with any legal bus I/O operation. The noise margin available in Single-ended is large enough that hot swapping is guaranteed to work.

Differential Bus

When inserting differential drives into an active bus, it is necessary to treat the +/- signals as totally independent. When doing a mechanical insertion of a SCSI device, the time skew between top and bottom rows of contacts mating can exceed a millisecond. The data transfer period in synchronous operation is 50ns (Fast-20) or 25ns (Fast-40), which means that we cannot expect the + and – pins to mate within the same synchronous cycle. Most SCSI noise sources appear as common mode noise, and are rejected by the differential circuitry. Hot swap glitches appear as a differential noise source, and the differential circuitry has less noise margin than the single-ended circuitry.

In a differential configuration, the ground loop between the switched capacitor and the bus connection is so long (in time and distance) that it is not really involved in the glitch. The tight coupling of the differential lines maintains the differential voltage across the bus, and the transient gets propagated to the terminator as a common mode disturbance.


Hot Swap in Multi-Mode SCSI

Devices in this category can operate in either SE (Single-Ended) mode, or in LVD (Low Voltage Differential) mode. Hardware and firmware contained within the Quantum Atlas III and Viking II drives monitor a single wire on the SCSI bus (DiffSense) to determine the modes of the other devices on the bus. Then the firmware configures the SCSI interface circuitry to match the mode prevailing on the bus.

Both Quantum Atlas III and Viking II drives are Multi-Mode SCSI disk drives. Among the advantages of a Multi-Mode disk drive is the flexibility to fit into either legacy SE systems (installed base), or into leading edge LVD systems running at double the bandwidth. This enables:

  • One part number for add-on sales to both the installed base and for brand new OEM sales
  • Sales of new high capacity (18GB) disk drives into installed base equipment which can later be migrated to LVD systems being introduced this year
  • Lower inventory expense

Multi-mode SCSI capability on disk drives can be a significant advantage on a system with a differential bus, when archiving or backup is required as tape drives with LVD are currently not available. Single-ended devices such as tape drives can be added to the bus using Quantum Atlas III or Viking II disk drives for backup purposes. Single-ended tape drives are not hot swappable.


Applicable SCSI Physical Documents

If additional information is required, refer to the following documentation:

  • SCSI-3 SPI (SCSI Parallel Interface), revision 15, now ANSI Standard No. X3.253.1993
  • Device Insertion/Removal classes
  • SCSI-3 Fast-20, revision 6, now ANSI Standard No. X3.277.1996
  • Documents 20 Mega-transfer/second operation
  • Defined active negation
  • SCSI-3 SPI-2 draft standard, revision 19
  • Defines LVD signaling
  • Defines Multimode (LVD/SE) transceivers
  • Defines LVD active terminators
  • Defines Multimode (LVD/SE) terminators
  • Defines new functionality for the DIFFSENS pins
  • Documents Fast 40 transfer rate operation
  • Documents LVD connector pin assignments for LVD buses
  • This document is a superset of SCSI standards, incorporating contents of both SPI and Fast-20

See Also