This bug has been fixed for Release 14 (R14). For previous releases, please read below for any possible workarounds:
This has been verified as a bug in Simulink 5.1 (R13SP1) in the way that Simulink handles connections between two subsystems at the same asynchronous rate. An error is thrown, but this error is unnecessary.
Currently, to work around this issue, add a Rate Transition block between the two models. "Ensure data integrity during data transfer" can be unchecked to eliminate any delays. The data transfer will be deterministic as long as the subsystems are guaranteed to be at the same asynchronous rate.
