axis changes from + to - by itself

[xaco]

hello everyone,i have a problem that occurs sometimes but ruins all...   it happened that i wanted to stop milling process and i did it with "feed hold" button...but after i started milling again with "cycle start" it happened that both x axis and y axis switched + and - ,so milling  continued in opposite,wrong direction.            this problem occurred also when i would move my tool with jogging,for example i push "up" button but it goes down,and after few tries it goes up as it should be...  whats happening??

[Tweakie.CNC]

If your controller is not recognizing the direction signals correctly then the motors could rotate in the wrong direction. This situation can occur when the controller is expecting the full 5 volt TTL signals for it's inputs but the computer's LPT parallel port is only outputting 3.3 volt TTL signals. In many cases this depends on the operating threshold of the opto isolators used in the breakout board so it may be a good idea to measure your parallel port direction pin voltage and and if it is not the 5 volts TTL standard (0 volts when jogging one way and 5 volts when jogging the other way) check that your BoB is compatible.

Just a thought.

Tweakie.

[xaco]

great answer Tweakie,i guess thats it,just in that case the change of direction should always occur in same direction..instead of going -x and -y it switches to +x and +y i guess, and never it could switch from +x to -x cos for that we need 5 volts impulse...i guess. So do you know where in mach i can see for what + or - direction i need those 5 volts?...anyway..this can be answer to my old question"why my mach suddenly go crazy" cos i experienced that doing parallel finish  sometimes instead of continuing going back and forth ,the tool continues to go forth and breaks...again as i remember ,it continued to +y instead of getting back to -y(which would obviously need 5 volts that he didn't get). thx