Semi E24
SEMI E24 defines Cluster-Tool Isolation-Valve Interlocks, the standard that specifies how the gate valves separating a cluster tool's central handler chamber from each attached process module must be electrically and logically interlocked with the robot's motion control, so the handler's arm can never extend into a module while that module's isolation valve is closed. A cluster tool's modules routinely run at different pressures and process conditions than the shared transfer chamber, and the isolation valve between them is the only thing protecting both the module's process environment and the robot arm itself from a collision or a pressure/contamination event — E24 is what guarantees that protection isn't just a good practice left to each vendor's judgment, but a wired, verifiable interlock every compliant cluster tool must implement the same way.
The critical design choice E24 standardizes is that the interlock must read the valve's actual physical state rather than trust the last command the control system issued to it — a valve commanded to open can still be physically stuck closed due to a mechanical fault, and a software-only interlock that merely checks "did I tell it to open" would let the robot extend into a closed valve exactly in the failure scenario where the interlock matters most. This is why E24 is classified as a safety standard rather than a sequencing convenience: a cluster tool without a hardware-verified isolation interlock can be made to look correctly sequenced in software while still allowing a genuinely dangerous collision between the end-effector and a valve that silently failed to open, which is the class of latent fault E24's hardware-state requirement is specifically designed to close off.
E24's interlock sits directly on top of the mechanical guarantees E21 and E22 already establish: those standards ensure the robot physically fits and can reach any compliant module without colliding with fixed hardware, while E24 ensures the one moving obstruction between the handler and each module — the isolation valve itself — can never be the thing the robot collides with either.