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Invalid characters error when trying to use "IF" statement in GCode

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05 Mar 2023 00:36 #265879 by xph1l
Invalid characters error when trying to use "IF" statement in GCode was created by xph1l
Hi,
My very first post on this forum. Also pretty much my first CNC build - wasn't easy to set everything up in LinuxCNC but I just LOVE it.
I'm using LinuxCNC v2.9.0-pre0 with QtPlasmaC v1.233.249.

The issue I'm having is that when trying to use "if/else/endif" or "while/endwhile" statements in GCode, the QtPlasmaC throws an error "Invalid characters, line has been commented out".

Quick look into the source (main branch) and I found that it is triggered in src/emc/usr_intf/qtplasmac/qtplasmac_gcode.py, in illegal_character() function.
The code that triggered this is:
    elif line[0].isalpha():
        # line starts with two alpha characters (this could be refined)
        if line[1].isalpha(): return 1

This check was committed by:
commit 07f8e0af364e7c9594084e2c2b629f3bc45288bf
Author: Phillip Carter <phillcarter54@gmail.com>
Date:   Wed Dec 8 14:22:01 2021 +1100

    qtplasmac: gcode filter
    
    fix cut recovery bug
    add more illegal character checks

My question is: was this behavior intentional, or was it an error? If intentional, then why?

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05 Mar 2023 00:38 #265880 by phillc54
Replied by phillc54 on topic Invalid characters error when trying to use "IF" statement in GCode
Can you post the gcode.
 

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05 Mar 2023 00:46 #265881 by xph1l
Replied by xph1l on topic Invalid characters error when trying to use "IF" statement in GCode
Sure thing. The "if" statement at the end of pre-amble is the culprit.
;     file name: mill_hook_small.tap
; date and time: Sat Mar 04 2023    09:09:11 PM
; postprocessor: LinuxCNC_PlasmaC_Mod_v1.scpost rev: 0.2
;                                  
;begin pre-amble                   
 G21 (units: metric)              
 G40 (cutter compensation: off)    
 G90 (distance mode: absolute)    
 M52 P1 (adaptive feed: on)       
 M65 P2 (enable THC)               
 M65 P3 (enable torch)             
 M68 E3 Q0 (velocity 100%)       
 G64 P0.254 Q0.025 (tracking tolerances: 0.254mm)
                                   
 if [#5400 eq 0]                  
  M30                              
 endif                             
                                   
;end pre-amble                    
;                                 
;begin material setup             
 T0 M6 (select plasma tool)       
 G43 H0 (apply tool offsets)       
 F#<_hal[plasmac.cut-feed-rate]>  
 #<holes> = 2 (over-cut for small holes)
 #<oclength> = 3.5 (over-cut length)
;end material setup               
 G0 X12.8 Y16.488                
 M3 $0 S1 (plasma start)           
 M67 E3 Q100                      
 G1 X9.901 Y19.387                 
 M67 E3 Q60             
 G3 X9.901 Y19.387 I2.899 J-2.899  
 M67 E3 Q100                            
 M5 $0 (plasma end)

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05 Mar 2023 00:51 #265882 by phillc54
Replied by phillc54 on topic Invalid characters error when trying to use "IF" statement in GCode
You need to use o codes in that case.
linuxcnc.org/docs/html/gcode/o-code.html
e.g.
 o100 if [#5400 eq 0]
  M30
 0100 endif

You can use names instead of numbers:
o<check_m30>  instead of  o100

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05 Mar 2023 01:07 #265884 by xph1l
Replied by xph1l on topic Invalid characters error when trying to use "IF" statement in GCode
Awesome, it works! Thanks!

Sorry for bothering, but I've brushed over the o-codes page and haven't found the reason for having all conditionals under o-codes and not allowing to use IF as-is.
Maybe you know the reason on top of your head?

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05 Mar 2023 01:10 #265885 by phillc54
Replied by phillc54 on topic Invalid characters error when trying to use "IF" statement in GCode
Personally I don't know. LinuxCNC follows the RS-274 gcode standards so it is probably part of that standard.

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05 Mar 2023 01:13 #265886 by xph1l
Replied by xph1l on topic Invalid characters error when trying to use "IF" statement in GCode
Got it. Will read up on that. Thanks for the help!

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06 Mar 2023 23:53 #265992 by andypugh
Replied by andypugh on topic Invalid characters error when trying to use "IF" statement in GCode

Sorry for bothering, but I've brushed over the o-codes page and haven't found the reason for having all conditionals under o-codes and not allowing to use IF as-is.

It's partly because of the way G-code consists of single-character commands. The I of "If" is only valid after certain G-codes, but the "F" could be a feed rate command. 

It's also partly because the parser doesn't have a loop / conditional stack to keep track of which IF belongs to which ENDIF. So the numbered O-words mean that if the condition fails the parser just needs to scan through the G-code waiting for the matching O else or O endif.  
The following user(s) said Thank You: phillc54

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07 Mar 2023 22:43 #266081 by xph1l
Replied by xph1l on topic Invalid characters error when trying to use "IF" statement in GCode
Oh, it makes sense that O-words make parser less complicated. Thanks for the explanation!

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