l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：
它有两组，组用以实现启动、停止控制。第二组仅一个END指令，用以结束程序。
l梯形图与助记符的对应关系：助记符指令与梯形图指令有严格的对应关系，而梯形图的连线又可把指令的顺序予以体现。一般讲，其顺序为：先输入，后输出（含其他处理）；先上，后下；先左，后右。有了梯形图就可将其翻译成助记符程序。上图的助记符程序为：
反之根据助记符，也可画出与其对应的梯形图。
VALMONT ELECTRIC 8G1028WF RAPID START BALLAST NIB
SYLVANIA MB2X40/120 RS MAGNETIC BALLAST NEW
ADVANCE REL-2P60-S INSTANT-START ELECTRONIC BALLAST NIB
VALMONT ELECTRIC 8G1148W RAPID START BALLAST
SIEMENS 3RT1044-1BB40 SIRIUS CONTACTOR
GRACO 220 395 DISPLACEMENT PUMP REPAIR KIT, NEW
ERDCO 3464-08T5 WATER ARMOR-FLO WATER METER, NNB
NORDSON 239819E SPARE PART KIT, NIB
PAPST 4482 FM FAN
TURCK PKW 3M-6/S90 PICOFAST CABLE NEW
TURCK BI 2-CRS260-AP4X2-H1141/S34 PROXIMITY SWITCH NEW
TURCK PKG 3M-2/S90 PICOFAST CABLE NEW LOT OF 2
SQUARE D ARC-13 30 AMP DISC SWITCH NIB
ALLEN BRADLEY 700-RTA / 700-RTA00V000A1 RELAY
NEC NL6448AC33-59D 10.4″ FLAT PANEL DISPLAY
S. HIMMELSTEIN 6-201 TRANSDUCER AMPLIFIER
SCHAFFNER FS5113-50-53 EMC FILTER
ACE CONTROLS MA 6450 INDUSTRIAL SHOCK ABSORBER NIB
HONEYWELL V5011G 1194 1 2-WAY SINGLE SEATED STEAM VALVE
KINETROL 059-404EL0000M VANE ACTUATOR & FILTER 1/4″ NIB
GAST DAA-V141-GB VACUUM PUMP 115 VOLT 5.5 AMP
SANYO ELECTRIC C-Q40L6C HERMETIC COMPRESSOR NIB
TURCK WKM 36-10M MINIFAST 3-WIRE CORDSET 600V 9A NEW
IPC AUTOMATION 910-8070-001 MICROWATCH ALARM PROCESSOR
ALLEN BRADLEY 1771-A3B1 12 SLOT I/O CHASSIS
MASS-TRON II 2 MASS FLOW TRANSMITTER 24 VAC/DC
INVENSYS 043 SERVICE REGULATOR, NIB
ALLEN BRADLEY 1756-CNB/D F/W REV 5.22 ControlNet MODULE
ALLEN BRADLEY 1756-CNB/A REV B01 F/W 1.14 ControlNet
ALLEN BRADLEY 1756-CNB/B REV C01 F/W 2.24 ControlNet
ALLEN BRADLEY 1756-DNB/B F/W Rev 7.002 DeviceNet MODULE
ALLEN BRADLEY 1756-ENET/A REV H01 F/W 1.18 ETHERNET
ALLEN BRADLEY 1756-ENET/A REV K01 F/W 1.18 ETHERNET
ALLEN BRADLEY 1756-CNB/A REV C01 F/W 1.19 ControlNet
ALLEN BRADLEY 1756-DNB/A F/W 3.3 REV F01 DeviceNet
ALLEN BRADLEY 1756-DNB/B F/W Rev 6.002 DeviceNet MODULE
ALLEN BRADLEY 1756-ENET/A REV L01 F/W 1.18 ETHERNET
HONEYWELL BAF1-2RN8-LH MICROSWITCH NIB
LEUZE ELECTRONIC RK 85/7 PHOTOELECTRIC SENSOR
HTC 222310D PCB
RELIANCE CORP EA100 LEVALARM
ENIDINE NB9599 HYDRAULIC SHOCK ABSORBER
MOORE 341GE1S2B1NNN13 TRANSMITTER
OMRON 3Z4SP-C22 VISUAL POSITIONING SENSOR
HUBBELL LED1-MR-RBB PATHFINDER LED EXIT NIB
ALLEN BRADLEY 1771-AL I/O ADAPTER MODULE
CUTLER HAMMER AN16BN0 SIZE 0 STARTER W/ C320KGT1
CLINCH NUT PULL TEST KIT, RETENTION TESTER 0-9000 LBS
LH RESEARCH MM11-3/115 MIGHTY-MITE POWER SUPPLY 375W
ENDRESS & HAUSER PMP41-PC13S6J11GC PRESSURE TRANSMITTER
BANNER PVA100P6EQ PVA SER 100MM ARRAY EMITTER KIT NIB
ACTARIS MODEL 600 FLOWMETER REGISTER
JAMFLOW 1/4″ BALL VALVE & WIKA PROCESS GUAGE NNB
KEYSTONE 276-020-400 AR2 BUTTERFLY VALVE 175 PSI NEW
VICKERS 508173 HYDRAULIC VALVE 24 VOLT COIL FOR DG4V
NEWMAN 45-313321 6″ BUTTERFLY VALVE 200 PSI NEW
DWYER VA1243 SERIES VA VARIABLE AREA GLASS FLOWMETER
PRESSURE GUAGE 0-6000 BAR MAKE AN OFFER…
KEYSTONE 723-703-050-AR2 BUTTERFLY VALVE 175 PSI NEW
ASHCROFT B429S XFM6B TYPE B SERIES PRESSURE SWITCH
KEYSTONE 195-703-080-920 BUTTERFLY VALVE 150 PSI NEW
VISIOTEC 633901 VISIOSCAN PINHOLE INSPECTION LENS
VICKERS 507848 VALVE SOLENOID COIL
ALLEN BRADLEY 1401-N42 Series A FUSE CLIP KIT, NEW
ALLEN BRADLEY 1494V-FS60 TRAILER FUSE BLOCK KIT, NIB
BANNER SMBPVA5A PROTECTIVE BRACKET KIT NIB
ALLEN BRADLEY 1785-L11B PLC-5/11 SER E CPU MODULE
BANNER SMB3212F MOUNTING ACCESSORIES LOT OF 22 NEW
KELTECH INC 193X642ABG01 RELAY BOARD
EDWARDS 104SLEDG-G1 ADAPTABEACON
ITC VLC10 LR 106309 FUSE BLOCK HOLDERS, LOT OF 4
PILZ PNOZ XV2 24VDC SAFETY RELAY
SIEMENS 3RK1200-0CE02-0AA2 MODULE
ALLEN BRADLEY 800E-2DL3 FULL VOLTAGE MODULE W/ LATCH
ALLEN BRADLEY 800MR-Q24BS PILOT LIGHT NIB
ALLEN BRADLEY 1490-N9 3/4INCH CONDUIT HUB NIB
PARKER 932670Q 10Q WN FILTER ELEMENT, NIB
SIEMENS 6ES7 132-4BD31-0AA0 OUTPUT MODULE
SIBA 20 001 04 FUSE LOT OF 3
