PLC stands for “Programmable Logic Controller”.

PLC controller can load and run small binary programs (PLC procedures). PLC procedure should be written in a simplified C-like language, compiled and stored in PLC controller memory to be ready to run. myCNC software includes PLC Builder - tiny IDE (Integrated Development Environment) to create and modify PLC procedure source files, compile them to binary code and upload it as RomFS iso image disk to PLC controller memory.

myCNC control has 2 types of built-in PLC systems that named Hardware PLC and Software PLC.

Note that hardware PLC can be launched from software PLC if necessary, through the use of gvarset(100040,HARDWAREPLC); command (replace HARDWAREPLC with your M-command of choice). You can read more on the process in the Software PLC section down below.

“Hardware PLC” means PLC system runs inside the CNC control board and able to access directly to CNC controller peripherals (inputs, outputs, PWMs, DACs, ADCs etc). PLC has also API to access to Motion Controller of a myCNC control board, so positioning commands are possible from Hardware PLC.

Hardware PLC is a tiny virtual machine that runs pre-compiled PLC procedures. PLC procedure can be started from

• G-code program M-code
• On-screen button
• Input Pin
• From TCP Ethernet Socket through Server API

Hardware PLC loop cycle time is 1ms. PLC core runs PLC procedure until the end of next loop. At the end of each loop PLC sleeps for 1ms, then continue running PLC. N-times repeat loop (even empty) will be executed N milliseconds. For example, delay for 10ms can be programmes as

timer=10; do{timer--;}while(timer>0);

IMPORTANT.
Hardware PLC is single-tasking. Only one PLC process can be executed at the moment. 
If new PLC procedure loaded, previous PLC process will be terminated immediately 
and replaced by the new one.

• PLC operates with 32 bits integer values only. Floating point operations don't work in PLC (typically need to use coefficient conversions to allow for more granular control).
• There is no need to declare variables. There is very limited space of 32 elements for variables.
• There a number of pre-defined variables in the PLC
  • eparam - External Parameter variable. A short manual is available here: Eparam
  • proc - variable is used to identify a process running in the PLC.
    The value is sent to the CNC control software and can be used to display Current PLC state (like Idle, Ignition (for plasma cutting), preheat (gas cutting), Tool change, Probing etc)
  • timer - value can be used as time counter inside PLC procedure
  • vexit - variable contains additional exit code. In case of error CNC control software can display Error message depends on vexit exit code.
  • message. The message variable is handled every loop delay time. If message value is not “0” PLC controller sends to CNC control software message code and reset the variable to “0”.
  • pwm01 - variable mapped to PWM #0 register. Writing to this variable will change PWM #0 value.
  • pwm02 - variable mapped to PWM #1 register. Writing to this variable will change PWM #1 value.
  • pwm03 - variable mapped to PWM #2 register. Writing to this variable will change PWM #2 value.
  • pwm04 - variable mapped to PWM #3 register. Writing to this variable will change PWM #3 value.
  • dac01 - variable mapped to DAC #0 register. Writing to this variable will change DAC #0 value.
  • dac02 - variable mapped to DAC #1 register. Writing to this variable will change DAC #1 value.
  • adc01 - variable mapped to ADC #0 register. Reading this variable will return ADC #0 value.
  • adc02 - variable mapped to ADC #1 register. Reading this variable will return ADC #1 value. Variables adc01, adc02 are a bit obsolete. It's better to use gvarget, gvarset functions and The hardware access registers to access to the control board peripherals.

PLC is a tiny virtual machine with a very limited register and memory space. 
PLC was created for very simple Inputs/Outputs manipulation. 
Heavy algorithms cannot be handled by PLC.
Please keep in mind this while creating Hardware PLC procedure.

• PLC operators

+

c=a+200;

-

c=a-150;

*

c=a*b;

/

c=b/7;

&

c=a&7;

|

c=b|1;

>>

c=a>>16;

<<

c=1<<n;

==

if (a==20) ......

!=

if (a!=0) ......

>

if (b>0) ......

>=

if (a>=0xf) ......

<

if (c<10) ......

<=

if (a<=b) ......

++

x++;

--

a--;

• PLC functions
  • g0moveA - send positioning command for PLC to the Motion controller.
  • portset - set to “1” selected output pin
  • portclear - clear to “0” selected output pin
  • portget - return selected input pin state (“0”, “1”)
  • gvarget - return Global Variable Register value. This function sends an inquiry about Global variable value to myCNC control software running on Host PC and waits till reply received. Waiting time can be 20…200ms depends on Host PC Operating System (Windows, Linux).
  • gvarset - set Global Variable Register to given value. This function sends to myCNC control software inquiry to change Global Variable Register to given value. There is a number of “Mapped” register addresses besides of “real” Global Array registers. List of Mapped addresses is shown in a table below.

Address	Description
20000…20100	Print variable value in myCNC control message widget for debugging purpose. Values written to this registers will be printed in myCNC control software in Message widget

It is possible to access the state of the output via gvarget commands from within the PLC process:

a=gvarget(0x400); //OUT0
b=gvarget(0x407); //OUT7

Starting from 0x400 to represent OUT0, this is a hexadecimal system that is simple to convert to dotted decimals (through the likes of a simple reference site here). Thus, for example, gvarget(0x40d); will return the state of Output #13.

• Running Motion commands from PLC
• Hardware PLC Examples
  • Getting a Height Map
  • M07 Mist Coolant ON
  • M03 Simple Spindle ON procedure
  • M88 M89 Stop Motion from PLC if Input pin activated
• Gas Cutting Control implementation
• API to work with Modbus devices from PLC
• THC API

“Software PLC” means PLC system runs inside the myCNC software and controls CNC controllers peripherals through Software API.

The main advantage of Software PLC is multitasking. All PLC procedures are running simultaneously and independent from each other.

Software PLC cycle time is 100ms, so Software PLC is suitable for wide range of slow applications like automatic lubricant control, fume exhaust control, alarm sensors control etc.

• All Software PLC procedures (except “system” procedures) are compiled and started simultaneously in separate threads with myCNC software start or after “Build All” button pressed in PLC Builder/Software PLC.
• “System” PLC procedures are procedures with names start with
  

  __ (double underscore)

  symbols. “System” procedures are not started automatically with the myCNC software start, but instead can be started automatically with some events or manually. There are a few pre-defined “System” PLC handlers-

__HANDLER_INIT

__HANDLER_EXIT

__HANDLER_GCODE_START

__HANDLER_GCODE_STOP

__BV17

Variables used in Software PLC:

• Software PLC Examples
  • How to add mandatory Homing after Emergency Button and-or Servo ready alarm
  • Button to toggle select output pin with indication
  • Oil Change counter
  • Controller Peripherals Test - BV17
  • Charge Pump

Language Core for Software and Hardware PLC are almost the same so language syntax is pretty the same and similar to C-language but VERY VERY stripped down.

• “32” - is a total number of variables PLC supports
• No need to define variables in source text. Variable name reserved when fist used. For example in line

  output0=15;

  PLC builder will define variable “output0” and assign the value to “15”

• var00…var15 : This variables are initialised before PLC procedure start from values defined in plc-variables.xml file.
• proc : proc variable value can be displayed on myCNC screen with display name “display-plc-proc” and can be used to show current status of PLC procedure
• message : If message value set to non-zero value, PLC controller will send a message to myCNC Control Core or myCNC software with parameters defined in variables
  message,
  var00 (usually defined as command as well) and
  var01 (defined as parameter).
  This way PLC controller able to communicate with Motion Controller and Host Software. Message variable is handled and cleared by PLC controller while “loop” operation, so message variable assign usually followed by short timeout loop like

  timer=2;do{timer--;}while(timer>0);

  Examples:

  //send to myCNC software information spindle speed is "0"
  command=PLC_MESSAGE_SPINDLE_SPEED_CHANGED;
  parameter=0;
  message=PLCCMD_REPLY_TO_MYCNC;
  timer=2;do{timer++;}while (timer>0);

  //Probing for Plasma Cutting.
  g0moveA(0x0,0x4,0-30000);                  //Start move down
  do { code=gvarget(6060); }while(code!=0);  //Wait till Motion Controller confirm moving started
  do{
   code=gvarget(6060);               //check Motion Controller (MC) current status
   sens=portget(INPUT_IHC);          //and check Probe input 
   if (sens==0)
   {
     message=PLCCMD_LINE_STOP;       //Send to MC command to Stop current line and wait next command
     code=1;                         //Set flag to exit from the loop
   };
  }while (code==0);                  //Exit from loop is motion finished OR Probe sensor pressed 

Normally PLC procedure should return code 99.

exit(99);

In case Error happened PLC procedure may return an error code. MyCNC software will catch exit code and report about Error if the error code is in PLC exit codes list.

A number of Global variable addresses are mapped to Hardware Inputs/Outputs. PLC procedure can access the controller peripherals through GVarGet/GVarSet function. Addresses to access to controller hardware are listed below

The following video illustrates the process of creating a button to launch a software PLC command (a similar process can also be used for hardware PLCs):