Petriflow 102 - Part 4

Types of Arcs: Read, Inhibitor & Reset Arcs

💡 What you'll learn

Understand how different types of arcs extend the expressive power of Petri nets - particularly read arcs, inhibitor arcs, and reset arcs - and how they influence token flow and transition enabling in Petriflow.

Overview

Goal & Context

This final part of the Petriflow 102 lecture introduces special arc types beyond standard input/output connections. These arcs are crucial for defining conditions, constraints, and reset mechanisms within process models.

The section covers:

• Standard arcs and their role in token movement
• Read arcs - testing presence of tokens
• Inhibitor arcs - preventing firing under certain conditions
• Reset arcs - removing all tokens from connected places

Each arc type provides a different way to control workflow logic.

Standard Arcs Recap

Basic Arcs

Standard arcs in a Petri net consume tokens from input places and produce tokens in output places when a transition fires.

Arc Type	Direction	Behavior
Input arc	Place → Transition	Removes tokens when firing
Output arc	Transition → Place	Adds tokens when firing

These arcs define the basic causal flow of activities.

Read Arcs

Definition & Example

A read arc connects a place to a transition but does not consume tokens. It merely tests whether the place contains a required number of tokens before allowing the transition to fire.

Property	Description
Purpose	Checks if a token exists without changing marking
Symbol	Small circle or special arc indicator
Condition	Tokens ≥ multiplicity

Example scenario:

• A transition “Start Engine” has a read arc from place “Battery Charged”.
• It can fire only if the battery is charged, but firing doesn’t consume the charge token.

NOTE

In Petriflow simulation, read arcs act like catalysts - conditions that must hold true but don’t alter the state.

Inhibitor Arcs

Definition & Logic

An inhibitor arc prevents a transition from firing if too many tokens are present in a place.
It introduces negative conditions - “this task can only occur if something is not true.”

Property	Description
Purpose	Disables transitions when tokens ≥ multiplicity
Condition	Tokens < multiplicity
Common Use	Prevents overproduction or mutual exclusion

Example:

• Task “Cool Reactor” may fire only if temperature < 10 tokens (representing degrees or units).
• This models a less-than condition, while read arcs model greater-than-or-equal.

A special case occurs when multiplicity = 1: this checks for emptiness of a place.

NOTE

Originally, inhibitor arcs were introduced in classical Petri net theory to test whether a place is empty, and later generalized to handle thresholds.

Reset Arcs

Purpose & Behavior

A reset arc clears all tokens from a place when its connected transition fires - regardless of how many there are.

Property	Description
Purpose	Removes all tokens from connected place
When Used	To restart or abort processes
Condition	Independent of current marking
Effect	Sets place token count to zero

Example:

• In a multi-step workflow, a “Reset Process” task can be triggered to clear all progress and return to start.
• This simulates an emergency stop or full rollback of the process state.

NOTE

Reset arcs do not determine whether a transition can fire - only what happens when it does.
They remove all tokens in their connected places, representing variable multiplicity that depends on the current marking.

Comparative Summary

Arc Type	Condition	Effect on Tokens	Typical Use
Standard Arc	Requires tokens	Consumes/produces tokens	Normal workflow flow
Read Arc	Tokens ≥ N	No token change	Check condition (catalyst)
Inhibitor Arc	Tokens < N	No token change	Prevent over-enabling
Reset Arc	Always enabled	Removes all tokens	Reset or cleanup logic

Video

Watch Petriflow 102 - Part 4 (36:29 – End).

Visual steps

1️⃣ Read Arc Example

Transition checks presence of a token but doesn’t consume it.

2️⃣ Inhibitor Arc Example

Transition only fires when connected place is empty or below threshold.

3️⃣ Reset Arc Example

Transition clears all tokens from connected place upon firing.

Summary

Concept	Description
Read Arcs	Test conditions without changing token marking
Inhibitor Arcs	Disable transitions when too many tokens exist
Reset Arcs	Remove all tokens, resetting a place
Use in Petriflow	Enables conditional and restart behaviors within workflows

Takeaway:

Advanced arc types make Petri nets and Petriflow more expressive and realistic.
They support conditional control, resource constraints, and state resets, helping model complex workflow logic beyond linear task flows.

You now understand:

• How read arcs differ from standard arcs
• How inhibitor arcs express negative conditions
• How reset arcs manage process restarts
• Why these mechanisms are crucial in real-world process automation