PLC Programming


Biomass Gasifier

Picture of Human Machine Interface HMI for Biomass Gasifier

This was one of the most challenging PLC projects I have been involved in. I had to create a biomass gasification process that would burn different biomass cleanly and efficiently. Accomplishing this was a personal highlight for me. My PLC programming experience is ladder logic using contact language.


Modicon PLC TSX 3722

TSX 3722 Features:

  1. Analog inputs
  2. Counter inputs
  3. Memory Expansion Slot
  4. Comm Slot (I used this for MODBUS RTU but many other protocols are support by purchasing different PCMCIA cards)
  5. 6 slots for IO cards built in
  6. Expandable chassis (I added an extra section for addition IO)
Modicon PLC TSX 3722  

PL7 4.5 Graphcet Example

PL7 Graphcet has three sections that are scanned in the order below.

  1. Prl (Preliminary)
  2. Chart (Sequential Processing)
  3. Post (Subquential Processing)

After the three Graphcet sections are scanned then the rest of regular sections are scanned.

The "Sr" folder is where subroutine logic is stored to be called by your ladder logic in other sections.

PL7 Application Browser

PL7 Application Browser


The Prl section is typically used to gather IO states for use subsequent sections.

These always true ladders cause subroutines SR0, SR1 and SR2 to be scanned.

PL7 Graphcet Prl Section

PL7 Graphcet Prl



This is the first two rungs of subroutine SR0.

  • Rung 1: If the HMI Stop_key is pressed then set the Auto_stop memory bit. If the Auto_stop bit is set then the program will initiate the shutdown process in other sections.


  • Rung 2:
  1. If the HMI Start_key is pressed and the gasifier is not in the process of shutting down and its not being loaded and the HMI is displaying the appropriate page then set the Auto_start memory bit. If the Auto_start bit is set the program will initiate the plant start-up process in other sections.
  2. If the HMI Start_key is pressed and the plant is in a Standby state clear the Standby state memory bits.

PL7 Subroutine Example

PL7 Subroutine


A Graphcet chart is more than a visual representation of an automated process it also control the process flow of the automation application.

PL7 Graphcet Chart Components:

  1. Starting Step

PL7 Graphcet Starting Step

  1. Transitions

PL7 Graphcet Transition

  • A Transition's ladder must be true for the process to continue to the next step in the chart
  1. Standard Step

PL7 Graphcet Standard Step

PL7 Graphcet Chart Section

PL7 Graphcet Chart Section

A PL7 Graphcet step has three parts.

  1. Activation
  • Code here is scanned once when the step is first activated
  1. Continuous
  • The ladders here are scanned as long as the step is active
  1. Deactivation
  • Code here is scanned once before being deactivated

PL7 Graphcet Step

PL7 Graphcet Step Details

PL7 PID Example


The best way to contact Chad is by email at
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