What does a low tone, high tone, then a 1 second pause mean on the AEC v2?

Question

What does a repeating low tone, high tone, and 1 sec pause mean on the AEC v. 2?

Answer

This series of tones indicates that the CPU board is not communicating to an extension board over RS-232. Verify that the extension board connected to the RS-232 wire is powered up correctly and that the RS-232 wire is connected and wired properly.