SIS logic solver function
A rounded square with an inscribed diamond marks a Safety Instrumented System, SIS function, the protective layer that takes the process to a safe state when a trip threshold is crossed. Per IEC 61511, ISA 84 the logic solver is structurally separate from the BPCS hexagon.
Read one of your own drawings.
Drop a P&ID, instrument index, or schedule. Tagsight reads it to the tag and opens a workspace you keep when you sign in.
PDF · DWG · DXF · TIFF · PNG · XLSX
How it’s drawn.
A square with rounded corners holding an inscribed diamond. The diamond inside the square sets it apart from the regulatory-control hexagon and marks a safety function, the protective layer that drives the process to a safe state on a trip. Trip logic tied to a shutdown valve is drawn this way so safety scope reads apart from basic control at a glance.
Typical usage.
Trip functions, PSHH, LSLL, TSHH tied to an emergency shutdown valve route through an SIS logic solver. The rounded-square symbol disambiguates SIS from BPCS scope at a glance, which matters for cabinet planning, cabling separation, and proof-test discipline.
Telling it apart.
- The diamond is the safety marker. A plain hexagon is basic process control. The diamond inside a rounded square is the safety-instrumented layer.
- Keeping the safety symbol distinct matters because the safety function is meant to stay independent of the control it protects.
- A trip tag such as PSHH or LSLL routed to this symbol is part of the protective layer, not a normal control loop.