msc {
 hscale="1.5", arcgradient="5";

 a [label="EMC"], b [label="iocontrol"],c [label="HAL TC component"], d [label="Machine"];

 a=>b [label="M6"];
 b=>c [label="tool-change = 1",ID=1];
 c=>d [label="change tool"];
 c=>c [label="wait for change complete"];	
 c<=d [label="change complete"];
 b<=c [label="tool-changed = 1",ID=2];
 b=>c [label="tool-change = 0",ID=3];
 a<=b [label="M6 command complete",ID=4];
 b<=c [label="tool-changed = 0",ID=5];
}