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In particular I'd like to know what schemes PCI Express, PCI and ISA use at the physical layer.
artistoex
Posted 2011-12-05T19:17:07.367
Reputation: 3 353
How do home computer buses electrically put data on the wire?
Answers
1
They're all different. You can easily look up each one. PCI Express uses low-voltage differential signalling. PCI uses both 5 volt and 3.3 volt signalling levels. ISA uses simple TTL signalling.
David Schwartz
Posted 2011-12-05T19:17:07.367
Reputation: 58 310
so 1 and 0 zero are resp. coded as 5 V and 0 V signal levels in pci? – artistoex – 2011-12-05T19:40:29.067
PCI supports both 5V and 3.3V signalling. Most likely, the actual signalling rules are something like: For 5V signalling, 0 is less than .7 volts, 1 is more than 3 volts. For 3.3V signalling, 0 is less than .3 volts, 1 is more than 2 volts. – David Schwartz – 2011-12-05T19:43:19.293
wow, the ISA spec fits on a couple of pages. I wonder what the PCIe spec is in kilo grams. – artistoex – 2011-12-05T19:56:39.843
Regarding differential signaling: Is there a direct mapping of the levels of the regenerated difference signal onto 0 and 1 as with the unipolar scheme in PCI and ISA? – artistoex – 2011-12-05T20:18:42.250
The typical setup is that at least a .247 volt difference is required to indicate a legal value. The maximum a driver is permitted to output is a .454V difference. The average voltage of both differential outputs remains between 1.125V and 1.375V. Which line is at a higher voltage than the other determines if it's a 1 or a 0. (Of these buses, PCI-Express is the only one that is serial and the only one that uses differential signalling. You may need to double-check these values against different versions of PCI-Express, but it's about right.) – David Schwartz – 2011-12-05T20:36:43.227