The answer to your question is NO.
The PIPE specification is not part of the PCI Express Base
Specification. It is not even governed by the PCI-SIG. It is a
"suggestion" from Intel for a logical interface between the MAC and the
PHY, in the physical layer of a PCI Express component. I think the
idea is to suggest that people designing PHY functions use a common
interface, to foster portability between PHY vendors/implementations.
As you note, it describes the logical behavior of such an interface and
doesn't discuss anything else. At the time it was written, I don't
think they were even contemplating external PHY devices like the
Philips PX1011a. However, you can imagine that someone wanting to do
an external PHY would probably take the PIPE specification as a
reasonable starting point for their interface behavior. What
electricals to use? Whatever you want, as long as it works and you
think your potential customers will be satisfied. Philips used SSTL2,
as you point out, which seems to work well with, say, a Spartan3
FPGA
from Xilinx.
So, then, if you are using an external PHY, you need to evaluate your
options and then select one -- and design to the specifications in the
vendor's data sheet.
Eric Crabill
Xilinx, Incorporated