"STOP D"不一定是外部停，很多情况都有可能触发STOP D，
报警27021的内容如下：
AXIS x1 stop D triggered
按西门子手册：
Parameters: %1 = Axis number
Definitions:
This alarm is output together with alarms 27011 "Safe velocity exceeded" or 27012 "Safe limit position exceeded" (when configured as such in $MA_SAFE_VELO_STOP_MODE, $MA_SAFE_VELO_STOP_REACTION or $MA_SAFE_POS_STOP_MODE).
Reaction:
NC Start disable in this channel.Interface signals are set.Alarm display.NC Stop on alarm.Trigger a "Deceleration on the path" and internal activation of the safe operational stop (SBH) after expiry of the time set in MD $MA_SAFE_STOP_SWITCH_TIME_D.
Remedy:
Eliminate causes of alarm "Safe velocity exceeded" or "Safe limit position exceeded" (see des cription of these alarms).
从以上内容可以看出，这里的STOP D是由于在交叉数据比较时，NCK发现机床‘超出了速度限制’或‘超出了位置极限’造成的，而安全集成配置数据MD36961 $MA_SAFE_VELO_STOP_MODE, MD36962 $MA_SAFE_POS_STOP_MODE或MD36963 $MA_SAFE_VELO_STOP_REACTION设置了在以上二种故障状态下触发STOP D。