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There seems to be somewhat differing and sometimes overlapping definitions of these terms. Below are definitions found in The Automation, Systems, and Instrumentation Dictionary, Fourth Edition by ISA.

permissive

A condition within a logic sequence that must be satisfied before the sequence is allowed to proceed to the next phase. [ANSI/ISA-84.01-1996]

interlock

  1. To arrange the control of machines or devices so their operation is interdependent in order to assure that they are coordinated properly. [ISA-RP55.1-1975 (R1983)]
  2. A physical device, equipment, or software routine that prevents an operation from beginning or changing function until some condition or set of conditions is fulfilled.
  3. A device, such as a switch, that prevents a piece of equipment from operating when a hazard exists.
  4. To join two parts together in such a way that they remain rigidly attached to each other solely by physical interference.
  5. A device used to prove the physical state of a required condition and to furnish that proof to the primary safety control circuit.
  6. A device or group of devices (hardware or software) that are arranged to sense a limit or off-limit condition or an improper sequence of events. To avoid an undesirable condition, they then shut down the offending or related piece of equipment or prevent it from proceeding in an improper sequence. [ANSI/ISA-77.44.01 & .02-1995]

Definition 2 under interlock seems to satisfy the definition for permissive.

Is the term permissive distinct from the term interlock, or is it a type of interlock used as a start/step condition? Or are the terms used differently under different circumstances?

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  • \$\begingroup\$ from your description ... permissive is a logic term ... interlock is hardware \$\endgroup\$
    – jsotola
    Jan 7 at 21:08
  • \$\begingroup\$ @jsotola Points #2 and $6 of the Interlock description both refer to software as well as hardware \$\endgroup\$
    – Peter M
    Jan 7 at 21:25

2 Answers 2

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My thinking :

Permissive :
A condition within a logic sequence that must be satisfied before the sequence is allowed to proceed.

Interlock :
A device used to prove the physical state of a required condition and to furnish that proof to the primary safety control circuit.

Hence , the "Permissive" is a condition which can be checked by the "Interlock" to proceed.

Example 1 :
"Starting the Electric Device" requires the condition that [Permissive] "It must not be raining". There is a Water Detector [Interlock] which checks this and disables the Power Switch (with either software or hardware) when it is raining.

Example 2 :
"Starting the Centrifuge" requires the condition that [Permissive] "Contents must not exceed 10 kg". There is a Weighing Machine [Interlock] which checks this and disables the Power Switch (with either software or hardware) when weight exceeds the limit.

Example 3 :
"Aero-Plane Auto-Pilot landing" requires the condition that [Permissive] "runway must be clear". There is a Camera System [Interlock] which checks this and will not allow the landing attempt when runway is not free.

Example 4 :
"Moon Rover landing" requires the condition that [Permissive] "rover Battery must be at least 90% charged". There is a Charging System [Interlock] which checks this and will not enable rover landing operations when the Battery Charge is less.

SUMMARY :
The "Permissive" is a Condition.
The "Interlock" is a Device which enforces that Condition.

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This is not a definitive answer, but in practice I use the term "permissive" to describe something that allows a process to start and "interlock" as something that allows a process to continue.

This means the permissive can become FALSE after the process starts without affecting the process, but an interlock must remain TRUE throughout.

Both permissives and interlocks can exist purely in software (some process must be completed before another can start, two processes can't run at the same time), but often have a hardware component (such as a sensor).

An interlock may also exist only in hardware, as in definition #3 from your question.

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