crosstalk, not wire length

Posted Oct 2, 2009 0:21 UTC (Fri) by gus3 (guest, #61103)
In reply to: Log-structured file systems: There's one in every SSD by dlang
Parent article: Log-structured file systems: There's one in every SSD

> if 1 wire lets you transmit data at speed H, N wires will let you transmit data at a speed of NxH.

That is true, when the bus clock speed is slow enough to allow voltages and crosstalk between the wires to settle. However, as clock speeds approach 1GHz, crosstalk becomes a big problem.

> the problem with parallel buses at high speeds is that we have gotten fast enough that the timing has gotten short enough that the variation in the length of the wires ... and the speed of individual transistors varies enough to run up against the timing limits.

Wire length on a matched set of wires (whether it's cat5 with RJ-45 plugs, or a USB cable, IDE, SCSI, or even a VGA cable) has nothing to do with it. The switching speed on the transmission end can accomplish only so much, but there has to be some delay to allow the signal to settle onto the line. The culprit is the impedance present in even a single wire, that resists changes in current. The more wires there are in a bundle, the longer it takes the transmitted signal to settle across all the wires. By reducing the number of wires, the settling time goes down as well.

Related anecdote/urban legend: On the first day of a new incoming class, RAdm Grace Hopper would hold up a length of wire and ask how long it was. Most of the students would say "one foot", but the correct answer was "one nanosecond."

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crosstalk, not wire length

Posted Oct 2, 2009 17:15 UTC (Fri) by giraffedata (guest, #1954) [Link] (1 responses)

That's good information about transmitting signals on electrical wires, but it doesn't distinguish between parallel and serial protocols.

Crosstalk is a phenomenon on bundled wires, which exist in serial protocols too: each wire carries one serial stream. This wire configuration is common and affords faster total data transmission than parallel with the same number of wires.

Signals having to settle onto the line also happens in serial protocols as well as parallel.

Is there some way that crosstalk and settling affect skew between wires but not the basic signal rate capacity of each individual wire?

crosstalk, not wire length

Posted Oct 7, 2009 5:47 UTC (Wed) by gus3 (guest, #61103) [Link]

I had to think about the crosstalk vs. skew issue for a bit, but I think I can explain it. (N.B.: IANAEE; I Am Not An Electronics Engineer. But I did work with one for a couple years, and he explained this behavior to me.)

Take an old 40-conductor IDE cable, for example. Typically, it's flat; maybe it's bundled. Each type creates its own issues.

A flat cable, with full 40-bit skew, basically means that the bit transmitted on pin 1, can't be considered valid until the bit on pin 40 is transmitted, AND its signal settles. Or, with an 8-bit skew, bits 1, 9, 17, 25, and 33 aren't valid until bits 8, 16, 24, 32, and 40 are transmitted.

(IIRC, an 80-conductor cable compensated for this, using differential signaling, transmitting opposite signals on a pin pair, using lower voltages to do so. This permitted less crosstalk between bits, while speeding the signal detection at the other end. But I could be wrong on this.)

A bundled 40-conductor cable is a little better. Think about an ideal compaction: 1 wire in the center, 6 wires around it, 12 around those, 18 around those, and 3 more strung along somewhere. From an engineering view, this could mean bit 1, then bits 2-7 plus settling time, then bits 8-19, plus settling time, then bits 20-37 plus settling time, then bits 38-40 plus settling time. (This from an iterative programmer's mind-set. A scrambled bundle might be better, if an EE person takes up the puzzle.)

Now, consider a SATA bus. Eight wires: ground, 2 differential for data out, ground, 2 differential for data in, ground, and reference notch. Three ground lines, with the center ground isolating the input and output lines. Add to this the mirror-image polarity between input and output; the positive wires are most isolated from each other, while the negative wires are each next to the middle ground wire. The crosstalk between the positive input and positive output drops to a negligible level, and the negative lines, near the center ground, serve primarily for error checking (per my best guess).

I hope my visualization efforts have paid off for you. Corrections are welcome from anyone. Remember, IANAEE, and my info is worth what you pay for it. This stuff has been a hobby of mine for over 30 years now, but alas, it's only a hobby.