求各位老师指点

在使用STL步进指令时,在之前必须加个状态转换条件.X,T,或M.如
---II-----[SET S0]
M8000
---ISTL S0I-----II-------[SET S20]
                          S0
---ISTL S20I---------------(YO)
                         -------------(T0 K10)
                           T0
                         ---II---------[SET S21]
----ISTL S21I---------(Y1)
                          ----------(T1 K10)
                               T1
                          ----II-------(S0)
                          ---------------[RET]
先写状态转移图,再编写...
2楼的同志错了..RET是步进返回指令.....SRET才是子程序返回指令..