If a register has an IOB=TRUE property and is also area-constrained to a group of slices, the register is mapped to a slice and MAP displays the following warning message:
"WARNING:Pack:680 - The register symbol core/the_signoise/Dac has the property IOB=TRUE, but failed to join an I/O component. Please try constraining the register together with a valid pad or I/O buffer symbol."
The message is correct; however, it does not identify the conflict that prevents the IOB pack. There are no plans to modify this behavior. It is possible to override the AREA GROUP constraint with a LOC constraint to a specific IOB site.