crosstalk, not wire length

Posted Oct 7, 2009 5:47 UTC (Wed) by gus3 (guest, #61103)
In reply to: crosstalk, not wire length by giraffedata
Parent article: Log-structured file systems: There's one in every SSD

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.