# How do home computer buses electrically put data on the wire?

In particular I'd like to know what schemes PCI Express, PCI and ISA use at the physical layer.

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## 1 Answer

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.

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so 1 and 0 zero are resp. coded as 5 V and 0 V signal levels in pci? –  artistoex Dec 5 '11 at 19:40
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 Dec 5 '11 at 19:43
wow, the ISA spec fits on a couple of pages. I wonder what the PCIe spec is in kilo grams. –  artistoex Dec 5 '11 at 19:56
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 Dec 5 '11 at 20:18
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 Dec 5 '11 at 20:36