My Turn

Serial ATA

Andrew W. Hill - 2001.12.05

My Turn is Low End Mac's column for reader-submitted articles. It's your turn to share your thoughts on all things Mac (or iPhone, iPod, etc.) and write for the Mac web. Email your submission to Dan Knight .

A while back I wrote about SCSI. Personally, I love SCSI. Its intelligent, fast, and efficient. Standard parallel ATA is just plain dumb. With the new Serial ATA specification, however, they've given ATA more of a brain.

First of all, I'd like to voice my largest complaint about the Serial ATA protocol - it's being called "ATA/1500." They rate it at 1500 Mbps (which for some reason comes out to 150 MBps). This is just like FireWire being rated at 400 Mbps and USB 1.0 at 12 Mbps. And it's just the opposite of the MBps (megabytes per second) ratings for SCSI and UltraATA.

Why can't developers decide on a standard set of units? People are shouting about how much faster Serial ATA/1500 is than UltraATA/100. At first glance, it looks like its 15 times as fast, but you have to deal with the difference between bits and bytes. ATA/1500 is only 50% faster than UltraATA/100. This is still significant, but it involves confusing advertising methods.

Serial ATA is a point-to-point process. This is going even further away from SCSI than where ATA is now. Instead of having sixteen, or seven, or two devices (master and slave) per port, Serial ATA can only have one device per port. A controller card would have between two and six ports with a four pin connection for each. Hubs are not allowed. This is probably not a bad thing, as it is easy to troubleshoot bad cables and devices without worrying about device interactions.

The current parallel ATA cable uses 80 lines and a 40-pin connector. 26 of these 40 wires are data lines (the rest, presumably, being grounds). As the Serial ATA cables have four conductors, the cables are very thin. This means that it is feasible to have a separate cable for each device without the insides of the computer becoming cluttered with excess cabling. It also leads to better ventilation. In Macs this is less of an issue than in PCs, due to the positioning of the hard drives, but it still helps. For reference, the Serial ATA cable will be just a bit larger than an internal CD audio cable.

Serial ATA purportedly has a significantly improved DMA (direct memory access). Many of the intelligent functions that were unique to SCSI are in the specification. In addition, the star topology of the drives allows for reliable RAID systems to be implemented.

Despite this increased intelligence, Serial ATA should be available at the same price as parallel ATA. Due to its speed and increased intelligence and the continued low price, it is expected that Serial ATA could displace SCSI. Maxtor notes that there is no reason why ATA drives have lower physical specifications, other than those that want high performance drives usually want SCSI. With Serial ATA it is quite possible that there will be 15,000 rpm ATA/1500 drives on the market.

It is interesting to note the relationship between Serial ATA and FireWire. IEEE 1394 was in the SCSI-3 specification as one of the "alternate" SCSI types, along with Fiber-Channel. Serial SCSI has been around for a few years, and despite promised speeds of 150 Mbps (which were expected this year) they never arrived. Serial ATA has been listed as an internal-only protocol, like the current parallel ATA, so it won't be competing with FireWire for external devices.

If Serial ATA proves to be everything it promises, it may mean the demise of SCSI. Maybe it's worth it.


Andrew W. Hill (a.k.a. Aqua) has been using Macintosh computers since 1987 and maintains that the Mac SE is the perfect Macintosh, superior to all - including the Color Classic. He is on the verge of being evicted from the family home due to its infestation of Macs (last count: about 50). Andrew is attempting to pay his way through college at UC Santa Cruz with freelance web design and Mac tech support.

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