Shared display, shared control

How to read it

The square around the bubble marks an instrument that shares its display and control with others. Course notes by Peter Woolf and others set discrete instruments, which stand apart from other instruments, against shared display, shared control instruments, which share functions with them; in their example, a discrete flow transmitter sends its reading to a shared display, shared control instrument that shows the flow to the operator.

A distributed control system, or DCS, is one such shared system. Kuphaldt’s Lessons In Industrial Instrumentation describes it as many control computers, each running a handful of loops, networked together to share information with each other and with the operators’ display consoles. His chart gives the shared instruments the same lines as discrete ones: a plain square-and-circle in the field, a line across it in the main control room, dashed for the rear of the panel.

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How to read the tag letters

The letters in a bubble are the instrument’s tag, and the number under them is its loop. In Kuphaldt’s account of the ISA 5.1 system, the first letter is the variable the loop measures or controls, whatever the device itself senses: every instrument in a level loop starts with L, even a transmitter that infers level from pressure. The letters after it are the functions. So FIC is a flow indicating controller, PT a pressure transmitter, TE a temperature element and LT a level transmitter, and FC-135 is a flow controller in loop 135. The DOE’s handbook builds tags the same way, from the sensed parameter, then the indicator or controller type, then the type of component.

Look-alikes

Without the square, the circle is a single instrument: a field-mounted instrument or, with a line across it, a control room instrument. Kuphaldt’s chart also has a hexagon for a computer function and a diamond for logic; Woolf and others’ notes put programmable logic control in that last group.