New compile options and inverse time bug fix.

- Apparently inverse time motion were not working for quite some time.
Goes to show how many people actually use it. The calculation was bad
and is now fixed in this update. It should now work correctly.

- `;` comment type is now supported. This is standard on LinuxCNC and
common on 3d printers. It was previously not supported due to not
existing in the NIST standard, which is out-dated.

- New compile-option to ECHO the line received. This should help users
experiencing very weird problems and help diagnose if there is
something amiss in the communication to Grbl.

- New compile-option to use the spindle direction pin D13 as a spindle
enable pin with PWM spindle speed on D11. This feature has been
requested often from the laser cutter community. Since spindle
direction isn’t really of much use, it seemed like good good trade.
Note that M4 spindle enable counter-clock-wise support is removed for
obvious reasons, while M3 and M5 still work.

grbl/protocol.c

@@ -21,6 +21,11 @@
 
 #include "grbl.h"
 
+// Define different comment types for pre-parsing.
+#define COMMENT_NONE 0
+#define COMMENT_TYPE_PARENTHESES 1
+#define COMMENT_TYPE_SEMICOLON 2
+
 
 static char line[LINE_BUFFER_SIZE]; // Line to be executed. Zero-terminated.
 
@@ -33,6 +38,10 @@ static void protocol_execute_line(char *line)
   protocol_execute_realtime(); // Runtime command check point.
   if (sys.abort) { return; } // Bail to calling function upon system abort  
 
+  #ifdef REPORT_ECHO_LINE_RECEIVED
+    report_echo_line_received(line);
+  #endif
+
   if (line[0] == 0) {
     // Empty or comment line. Send status message for syncing purposes.
     report_status_message(STATUS_OK);
@@ -82,7 +91,7 @@ void protocol_main_loop()
   // Primary loop! Upon a system abort, this exits back to main() to reset the system. 
   // ---------------------------------------------------------------------------------  
   
-  uint8_t iscomment = false;
+  uint8_t comment = COMMENT_NONE;
   uint8_t char_counter = 0;
   uint8_t c;
   for (;;) {
@@ -101,14 +110,14 @@ void protocol_main_loop()
       if ((c == '\n') || (c == '\r')) { // End of line reached
         line[char_counter] = 0; // Set string termination character.
         protocol_execute_line(line); // Line is complete. Execute it!
-        iscomment = false;
+        comment = COMMENT_NONE;
         char_counter = 0;
       } else {
-        if (iscomment) {
+        if (comment != COMMENT_NONE) {
           // Throw away all comment characters
           if (c == ')') {
-            // End of comment. Resume line.
-            iscomment = false;
+            // End of comment. Resume line. But, not if semicolon type comment.
+            if (comment == COMMENT_TYPE_PARENTHESES) { comment = COMMENT_NONE; }
           }
         } else {
           if (c <= ' ') { 
@@ -121,9 +130,10 @@ void protocol_main_loop()
             // NOTE: This doesn't follow the NIST definition exactly, but is good enough for now.
             // In the future, we could simply remove the items within the comments, but retain the
             // comment control characters, so that the g-code parser can error-check it.
-            iscomment = true;
-          // } else if (c == ';') {
-            // Comment character to EOL NOT SUPPORTED. LinuxCNC definition. Not NIST.
+            comment = COMMENT_TYPE_PARENTHESES;
+          } else if (c == ';') {
+            // NOTE: ';' comment to EOL is a LinuxCNC definition. Not NIST.
+            comment = COMMENT_TYPE_SEMICOLON;
             
           // TODO: Install '%' feature 
           // } else if (c == '%') {
@@ -136,7 +146,7 @@ void protocol_main_loop()
           } else if (char_counter >= (LINE_BUFFER_SIZE-1)) {
             // Detect line buffer overflow. Report error and reset line buffer.
             report_status_message(STATUS_OVERFLOW);
-            iscomment = false;
+            comment = COMMENT_NONE;
             char_counter = 0;
           } else if (c >= 'a' && c <= 'z') { // Upcase lowercase
             line[char_counter++] = c-'a'+'A';
@@ -206,6 +216,8 @@ void protocol_execute_realtime()
         // to do what is needed before resetting, like killing the incoming stream. The 
         // same could be said about soft limits. While the position is not lost, the incoming
         // stream could be still engaged and cause a serious crash if it continues afterwards.
+        
+        // TODO: Allow status reports during a critical alarm. Still need to think about implications of this.
 //         if (sys.rt_exec_state & EXEC_STATUS_REPORT) { 
 //           report_realtime_status();
 //           bit_false_atomic(sys.rt_exec_state,EXEC_STATUS_REPORT);