The M6 command also has an output that can be enabled (a single pulse that comes on whenever the M6 command is issued. It stays on for a fixed amount of time). The manual states that the M6 command sends out a number of pulses equal to the tool number, but this is not correct. It sends out one pulse no matter what. I'm using the M6 command for toolchange, and the M73 to check for PLC input (when the ATC is done, it will tell TCNC that it can continue with the program).