1.Possible cause analysis of failure:
a.The limit switch is broken.
b.There is a problem with the limit switch wiring to the mainboard.
a.Check whether the limit cable is good or broken.
b.Judge whether the limt switch is good or broken.(The machine connected to the normally open contact and the table hit the buzzing block.A table pen is connected to the common end of the limit switch,another table pen is connected to NO.At this time the loop is impassable.The circuit is turned on when the limit switch iron pieces touch, and vice versa when released.)
3.Check whether the connection between the limit and the mainboard is normal.