What are the power differences between the CEI-520 and the CEI-520A?
Both the CEI-520 and the CEI-520A require 5V, +12V, and -12V. These voltages are mandated by the PCI specification, as is 3.3V. On both boards, Condor chose to create 3.3V on-card, as many PCs in use today fail to provide 3.3V to the PCI slot, although clearly required by the PCI Specification.
This is a different issue than the 'PCI Signaling Environment.' The PCI specification describes 5V and 3.3V (commonly referred to as 3V) PCI slots. Add-In cards may be 5V, 3.3V, or Universal. The connectors physically preclude plugging a card into an incompatible slot. This is readily apparent when looking at the card connector, as the Universal has two notches, and the 5V and 3V have one notch each.
To summarize, the CEI-520A is Universal PCI (5V and 3.3V), and the CEI-520 is 5V PCI only.