指令系统：一个PLC所具有的指令的全体称为该PLC的指令系统。它包含着指令的多少，各指令都能干什么事，代表着PLC的功能和性能。一般讲，功能强、性能好的PLC，其指令系统必然丰富，所能干的事也就多。我们在编程之前必须弄清PLC的指令系统
l程序：PLC指令的有序集合，PLC运行它，可进行相应的工作，当然，这里的程序是指PLC的用户程序。用户程序一般由用户设计，PLC的厂家或代销商不提供。用语句表达的程序不大直观，可读性差，特别是较复杂的程序，更难读，所以多数程序用梯形图表达。
l梯形图：梯形图是通过连线把PLC指令的梯形图符号连接在一起的连通图，用以表达所使用的PLC指令及其前后顺序，它与电气原理图很相似。它的连线有两种：一为母线，另一为内部横竖线。内部横竖线把一个个梯形图符号指令连成一个指令组，这个指令组一般总是从装载（LD）指令开始，必要时再继以若干个输入指令（含LD指令），以建立逻辑条件。后为输出类指令，实现输出控制，或为数据控制、流程控制、通讯处理、监控工作等指令，以进行相应的工作。母线是用来连接指令组的。下图是三菱公司的FX2N系列产品的简单的梯形图例：
750-403 WAGO DIGITAL INPUT, 4 CHANNEL 24V DC NEW IN BOX
BZ-2RW863-A2 MICRO SWITCH NEW
415-116-15 TOUGH GUN TREGASKISS LINER NEW
1492-GH100 ALLEN BRADLEY SINGLE POLE CIRCUIT BREAKER
1747-C11 AB PROCESSOR TO ISOLATED LINK CABLE NEW
871C-J4B12 ALLEN BRADLEY PROXIMITY SWITCH NEW IN BOX
700DC-P400Z24 ALLEN BRADLEY RELAY
509-TOD SERIES B SIZE 00 NON REVERSING MOTOR STARTER
90-102 STEVECO SWITCHING RELAY NEW IN BOX
V600-H11 OMRON AMPLIFIER READ/WRITE HEAD W/10M CABLE
53629-2 PHD MAGNETIC REED SWITCH NEW
700-HRM12TA17 ALLEN BRADLEY ON-DELAY TIMING RELAY
9012 GCW-1 SQUARE D PRESSURE SWITCH NEW NOT IN BOX
Z-25142 ALLEN BRADLEY COMPLETE OPERATING HEAD NEW
43007 BRAD HARRISON MINI-CHANGE RECEPTACLE NEW
EE-SPW311 OMRON PHOTO MICROSENSOR NEW IN BOX
30079 9000 SCHRADER BELLOWS QUICK EXHAUST VALVE NEW
1777-CA ALLEN BRADLEY I/O INTERCONNECT CABLE 3 FT. NEW
ALLEN BRADLEY 1770XYC LITHIUM BATTERY NEW 1 AVAILABLE
WATTS MIST LUBRICATOR. MODEL C528 02BLAHCA
PARKER CYLINDER, SERIES 2MA MODEL 02.00T2MAU14 2.000
B561ADA4 9C PARKER VALVE NEW IN BOX
B521BB549C PARKER PNEUMATIC VALVE NEW
925773 20C VF PARKER HYDRAULIC FILTER ELEMENT NEW
925773 20C NG PARKER HYDRUALIC FILTER ELEMENT NEW
872C-DH1NP8-E2 ALLEN BRADLEY INDUCTIVE PROXIMITY SENSOR
4436B61512060 FARADAY BELL W/6 GONG NEW
FUJI SG33B Earth Leakage Breaker
770-1 WHITE-RODGERS AIR SWITCH NEW IN BOX
700-PB40 ALLEN BRADLEY SECOND DECK CONTACT RELAY NEW
750-403 WAGO DIGITAL INPUT, 4 CHANNEL 24V DC NEW IN BOX
BZ-2RW863-A2 MICRO SWITCH NEW
415-116-15 TOUGH GUN TREGASKISS LINER NEW
1492-GH100 ALLEN BRADLEY SINGLE POLE CIRCUIT BREAKER
1747-C11 AB PROCESSOR TO ISOLATED LINK CABLE NEW
871C-J4B12 ALLEN BRADLEY PROXIMITY SWITCH NEW IN BOX
700DC-P400Z24 ALLEN BRADLEY RELAY
509-TOD SERIES B SIZE 00 NON REVERSING MOTOR STARTER
90-102 STEVECO SWITCHING RELAY NEW IN BOX
V600-H11 OMRON AMPLIFIER READ/WRITE HEAD W/10M CABLE
53629-2 PHD MAGNETIC REED SWITCH NEW
MKC-2 SPORLAN COIL ASSEMBLY WITH JUNCTION BOX NEW
41308 BRAD HARRISON MINI-CHANGE CORDSET NEW
804000A09M020 BRAD HARRISON MICRO-CHANGE CORDSET NEW
DCP-1.5W WENTECH POWER SUPPLY JUMP BOARD
1770XR ALLEN BRADLEY HIGH ENERGY LITHIUM BATTERY NEW
R-C NETWORK MODEL RCY6A-30V TRANSIENT VOLTAGE FILTER
MAC VALVES PME-111DAAG , 120VAC COIL PNEUMATIC VALVE
