Although MIL-STD-1553 has been around for almost 20 years and is well supported today, our experience in testing over the last five and a half years confirms the need for validation testing. Validation testing verifies the compliance of a terminal's interface with MIL-STD-1553. Testing is often avoided because of limited experience, increased costs and, most importantly, misconceptions about 1553. The misconceptions tend to be a mixture of hopeful expectations about compliance and misunderstandings about the performance of parts claiming to be “certified”, board designs that have been validated only in specific LRUs, and operational but untested LRUs.The complexity of MIL-STD-1553 provides many pitfalls for the unwary. Failures can result from marginal components, improper selection of components, incorrect usage of components, poor part layout and deficient software. A “certified” 1553 interface board from one LRU may fail the test plan when tested in a different LRU. LRUs may appear to operate correctly under normal conditions yet fail the test plan because they do not have the required margins. Without validation testing, the performance of a 1553 interface cannot be properly determined.