[Ham-Computers] RE: SATA

[Hsu, Aaron (NBC Universal)]

Hi Loren,

Yes, parallel data paths are quicker overall than a single serial data path, but parallel busses have inherent problems primarily related to timing.  When you send data down a parallel bus, you need to make sure that every bit arrives at the exact same time - this is where the difficulties are comming in.  As bus speeds get higher, the maximum trace/cable length shortens.  Also, slight capacitance differences in each "wire" of the cable can delay the signal propagation of that one wire just enough to cause a timing problem (there's a specific word for this, but I don't remember what it is...something about latching, I believe).  In the past, it was simpler to design a parallel bus to be faster because bus speeds were slower.

But, with today's technology, things are getting faster and cable lengths and tolerances are getting shorter/tighter.  It's already pretty difficult design parallel busses on motherboards because high bus speeds are bringing max trace lengths under 4 inches.  If you look carefully at DDR/2/3 memory modules, you'll notice that some traces zig-zag or spiral for no apparent reason - but this is to make sure that each bit arrives at the exact same time at the motherboard.

If I'm doing my math right, the max cable length for 0 wait states on a 100MHz bus (ATA-5, aka UDMA-100) is about 10 feet (assuming a 100% velocity factor).  So let's say 6 feet for ATA-6/UDMA-133.  Factor in cable VF and connector capacitance and the cable can now be no longer than 3 feet.  Use a 50% factor to ensure timing can be accounted for and the max length is now 18 inches.  Hey, guess what...that's the actual ATA-6 spec of 18" max!

Today's technology is able to run at very high speeds, but the inherent timing problems of a parallel bus now becomes a major roadblock.  Single-wire data-speeds of over 10GHz are possible, but trying to do this with a parallel bus is a nightmare.  So, using a serial bus and appropriate timing protocols, you can run a single wire at 10GHz (for example) and get the full 10GHz bandwidth out of that cable (minus overhead, of course).

Current SATA specs run at 1.5Gbps and 3Gbps (I believe they are baseband, so 1.5GHz and 3.0GHz).  This would be a nightmare with a parallel bus, but pretty easy on a serial bus.  This is the primary reason for the switch to serial architecture.  But guess what, it's possible to "parallel" these serial datapaths to get "aggregate" bandwidth - I believe this is called "link aggregation".  For example, the PCI-Express bus uses serial data lanes - but, you can combine up to 16 lines for higher bandwidth.  PCI-Express x16 (16-lanes) is used by video cards and you'll find PCI-Express x4 (4-lanes) on server motherboards for high bandwidth controllers (hard drive and network).  Why isn't this called a "parallel" bus?  Because each serial "lane" is still transfering data independent of the others and a controller re-combines the aggregated data after-the-fact.  In essence, it's a parallel bus...just doesn't work like a classic parallel bus.

Anyways, I hope this helps clarify things.  Serial buses are just easier to design at higher speeds.  But, in essence, you'll still find "parallel"-like architectures using multi-link serial connections.

If you haven't done so already, Wikipedia has a good article that covers some of this (though it doesn't really explain the technological hurdles of a parallel bus).  On the bottom you'll find a chart comparing several of the most popular hard drive interface busses - the chart includes max speed, cable length, and number of devices supported.

http://en.wikipedia.org/wiki/Serial_ATA

73,

  - Aaron, NN6O



________________________________

From: ham-computers-bounces at mailman.qth.net [mailto:ham-computers-bounces at mailman.qth.net] On Behalf Of Loren Moline WA7SKT
Sent: Tuesday, December 30, 2008 6:04 PM
To: ham-computers at mailman.qth.net
Subject: [Ham-Computers] SATA


Hello,

Can someone tell me why they have gone to Serial ATA drives?

How can someone say that serial could be as fast as parallel data transfer.

Just like USB external drives. No matter how fast USB is I can't believe it wouldn't be slower than parallel data transfer.

 
Loren   WA7SKT

 
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