• Instead of one common clock shared by all devices (old PCI/PCI-X 1.0),
  → the transmitter sends a strobe signal alongside the data.
• Both data and strobe travel the same physical path (same trace length, same delay).
• At the receiver:
  • The strobe is used directly to latch the incoming data.
  • This removes the uncertainty of clock skew and flight time delays.

✅ Result: The timing margin no longer shrinks when frequency increases → allows very high speeds.

Data Rates Supported

• DDR (Dual Data Rate): Data captured on both edges of the strobe → 2× throughput.
• QDR (Quad Data Rate): Data captured 4× per strobe cycle → 4× throughput.
• Bandwidth increased massively compared to PCI-X 1.0 (133 MHz). PCI-X 2.0 could reach 533 MHz effective speeds.

Why It Works Better

1. No clock skew budget → the strobe and data arrive together.
2. Flight time irrelevant → as long as both travel the same distance, the relative timing is preserved.
3. High-speed friendly → this is why you see similar ideas in DDR SDRAM and HyperTransport.

Limitations Introduced

• No shared-bus possible anymore

  • At such high frequencies, you can’t have multiple devices hanging off the same parallel bus.
  • Must be point-to-point connections.
  • More devices → more bridges → higher cost.

• Complex device design

  • A single device with 3 buses would need:
    • 3 full interfaces
    • An internal bridge
    • Very high pin count
  • Expensive silicon → targeted only at high-end servers/workstations.

Reliability Features

As PCI-X recognizes itself to high end market:

• ECC (Error Correcting Code) added on top of parity.
  • Can correct single-bit errors on the fly.
  • Detects multi-bit errors robustly.
• Important for enterprise servers, where uptime and reliability matter more than cost.