Technology

  1. Introduction
  2. FireWire 400
  3. FireWire 800
  4. Bridging
  5. Integrity Hot-Swap
  6. SCSI
  7. USB 2.0

Glyph uses the IEEE-1394 FireWire interface for interconnections within some of our products. Notable among these are the GT 103, GT 205 and GT 051 FireWire Hotswap units. Here we discuss a little about this Glyph innovation, Integrity™, which is at the heart of our most recent and advanced hard drive products.


The Parallel ATA Interface

Most hard drives are produced with the common parallel ATA/IDE interface, which was designed for fixed-drive use within a computer chassis. The ATA electrical signals, and the ATA software protocols, were never designed to support connection or disconnection of drives while power is applied. That action, called "hot-plugging", requires special electrical circuits and software, to manage the additional removal of devices, as well as defending against electrical problems when connectors are mated and demated.

Thus, the ATA interface is fundamentally unsuited to hot-plugging. This doesn't mean it can't be done, but it's basically trying to make the interface do something it was never intended to support. And that can cause problems for you in terms of data storage reliability.

FireWire vs. ATA as a Hot-Swappable Interface

As described on the FireWire Page, the IEEE-1394 interface is designed for exactly that kind of mate/demate application. So it was natural that removable drives would be built using FireWire as the interconnect, and portable FireWire hard drives became available to address this need.


But what of rack-mount equipment? Here, you have a rack-mount receiver bay, and you want to remove the drive in a front-panel tray or cartridge — not the entire rack. The rear-facing connections typically don't get unplugged.

Not surprisingly, "removable" hard drives appeared, which placed an ATA drive in a tray; the tray could be pulled out of a receiver bay and another tray swapped in. But these devices used the ATA interface as the tray/receiver interconnection. This was inexpensive, but meant that the trays could not be truly hot-swapped because the ATA interface and protocol don't support it.

In some products, an ATA-to-FireWire bridge circuit was placed in the rack unit, behind the receiver bay, and the bridge circuit attempted to sense the insertion and removal of the ATA drive (tray). It then translated these events into equivalent FireWire signals. The result could be described as a "FireWire HotSwap" product, but in fact the hot-swapping was being done at the ATA interface, not the FireWire interface. So all the problems of breaking the ATA interface remained. This was particularly unfortunate in rack-mount gear, because while the ATA interface was being broken at the front of the rack unit, there at the back of the rack was a FireWire interface, and it was always plugged in!

Although this hybrid was functional (and indeed Glyph sold a number of units with this type of setup in previous years), it was far from optimum. So Glyph took a look at this situation, and decided that somebody ought to do it right.

Glyph Integrity™ FireWire HotSwap Interface

So in mid-2002, Glyph engineering designed an interface based on the IEEE-1394a FireWire electrical interface, plus high-reliability SCA-II blind-mate connector technology used in high-end Fibre Channel and SCSI disk arrays, and developed a new interface which was designed from its conception for hot-swapping of hard drives.

The Integrity™ interface, first used in the Glyph GT-Series™ product line, incorporates a number of unique characteristics in addition to the use of existing industry standards. The Glyph FireWire HotSwap Interface has provisions for:

  • Two independent FireWire circuits (ports)
  • One USB 2.0 circuit (device-side "B" port)
  • 12V, 5V, and 3V power supplies
  • Power supply ramping (smooth-start) and current-limit controls
  • A range of sensing, signalling, and indicator circuits
  • Other features for future use

This interface supports hot-swapping of hard drives in the proper way — using circuits and protocols that were intended for use in hot-swap applications. As you might expect, Glyph's hot-swappable drive cartridges incorporate Glyph's ATA-to-FireWire bridging technology in the cartridge itself, so that when you remove the hard drive from the receiver, you're keeping the drive and its bridge together as a set. The bridge circuit in the Glyph cartridge was the first instance of this interface in a removable product.

Versatile, Expandable, and Compatible

Glyph's FireWire HotSwap technology was first implemented as a receiver on the interconnect backplane of the GT 308's 6-bay receiver chassis. There, each hotswap bay has its own FireWire port, to mate with the FireWire port on the cartridge. On the interconnect backplane, standard FireWire PHY chips serve to interconnect the receivers and channel the data to the FireWire ports on the back panel of the rack unit.

The next product to use Integrity™ was the FireWire HotSwap TableTop unit which has a single receiver port, but is otherwise identical to the Trip2's bigger interconnect. The HotSwap technology scales very well from single-bay units to configurations of almost any reasonable size — remember that FireWire can support up to 63 devices on a single bus!

As FireWire technology advances, the FireWire HotSwap will remain compatible. The recent development of IEEE-1394b (called FireWire-B or FireWire-800) doubles the data rates available over FireWire. Existing Glyph HotSwap receivers will support IEEE-1394b cartridges, and a 1394b version of the receiver interface will be incorporated into the products as well. A future Glyph technical White Paper will discuss the Glyph Integrity™ FireWire HotSwap Interface in greater detail, and describe how this interface can be incorporated into other products as well. Look to Glyph for innovation in high-reliability HotSwap Technology!