The protocols used in building PVM are evolving, with the result that binaries from newer releases are not compatible with older ones. Compatibility is determined by the pvmd-task and task-task protocol revision numbers. These are compared when two PVM entities connect; they will refuse to interoperate if the numbers don't match.
PVM releases with the same second digit in their version numbers (for example 3.2.0 and 3.2.6) will interoperate. Changes that result in incompatibility are held until a major version change (for example, from 3.2 to 3.3).