The TTU/515E or O/I Level Test Set was developed to provide a full end-to-end test of the NACES and FAST electronic sequencers used on the NACES ejection seat.  The O/ITS provides simulated inputs and measures the timing of initiator firing sequencer outputs.   

The O/ITS, via a serial channel in the test connector, performs discrete element diagnostics on all the sequencer's redundant electronics and verifies its full functionality. The O/ITS is designed with safety lockouts to prevent power application in the event all cables are not connected prior to test.

In the self-test mode, the O/ITS employs a complete wrap-around capability using the test cables and will fault isolate itself to the replaceable assemblies.  All interconnecting cables are stowed in a stowage compartment in the lid.

Qualified in accordance with MIL-T-28800C for Type II, Class 3 equipment, and EMI requirements of MIL-Std-461C for Class A-4 equipment.​