在300 PLC中，有如下语句，请高手解释：
A7d0: CLR
A #PULSE_ON
NOT
L #SELECT
L 2
= L 174.2
<>I
A #QC_ACT
O L 174.2
L #SELECT
L 3
= L 174.2
==I
O L 174.2
= #bLMNnew
L #SELECT
L 0
==I
L #SELECT
L 2
= L 174.2
==I
O L 174.2
A #PULSE_ON
= #bPulse
A #PVPER_ON
JCN A7d2
L #PER_MODE
L 1
==I
JCN A7d3
L #PV_PER
ITD
DTR
L 1.000000e-002
*R
T #rPVin
JU A7d4