PLC的基本工作方式是順序執行用戶程序，每一時鐘周期執行一條指令。對用戶程序的執行一般有循環掃描和定時掃描兩種，掃描過程分為三個階段，即輸入採樣階段、程序執行階段和輸出刷新階段，如圖1所示。
CARSON 34EP040-1194 40:1 PLANETARY GEARHEAD D586068
NORD SK92672N56C UNICASE 56C 37.32:1 WORM GEAR REDUCER D585052
HUB CITY 0220-86461 3206 UNIVERSAL POWER CUBE WORM GEAR REDUCER 200:1 D588060
NORD SK22 180TC2 7.57:1 GEAR REDUCER D586663
BOSTON GEAR SRF718-10N-B7-G WORM GEAR REDUCER 10:1 5/8IN X 1IN D584234
BROWNING CBN3483SV156U18?2TC 56:1 182TC HELICAL GEAR REDUCER D548930
COLOMBO FILIPPETTI CF3 130P-1.5-P244 GEAR REDUCER D549759
FALK 06UCBN2A2.5A1C ULTRAMITE HELICAL GEAR REDUCER 2.557:1 180RC D596596
THOMSON MICRON XDT115-005 DURATRUE 5:1 PLANETARY GEARHEAD REDUCER D530095
MASTER M94419F 56CM21A WORM GEAR REDUCER 1HP 192:1 D591817
SEW EURODRIVE K87AD3 56.64:1 WORM GEAR REDUCER D586929
TEXTRON B06-57630 CONE DRIVE 20:1 WORM GEAR REDUCER D591065
PARKER PS142-070-S2 70:1 PLANETARY GEARHEAD REDUCER D570644
STERLING S2175FHQ0405610?2 WORM GEAR REDUCER 0.469HP 40:1 D570775
A FISCHER G-2000 PHASE DRIVE GEAR REDUCER 5HP 4:1 D570773
PARKER PX115-015-F01S2 STEALTH PLANETARY GEARHEAD 15:1 D570873
DODGE CD318 11.6HP 18.19:1 SHAFT MOUNT GEAR REDUCER D547695
NORD GEAR SK 9033.1AZD 56C0.5 GEAR REDUCER 1361.37:1 D577949
NORD GEAR SK4382AZ N140TC 390.76:1 WORM GEAR REDUCER D578132
RELIANCE M054921003TS 180DM21A MASTER XL GEAR REDUCER 17.1:1 1750RPM D581831
DODGE M94831N 180DM21A GEAR REDUCER 4.1HP 1-3/8IN 1750RPM 102RPM D581833
SEW EURODRIVE K77AM184 25.62:1 WORM GEAR REDUCER D580243
BROWNING CBN2202SB3APNM HELICAL GEAR REDUCER 7.1:1 1980IN-LBS D593064
BROWNING CBN2202SB3APISN?M HELICAL GEAR REDUCER 1790IN-LBS 6.3:1 D593555
BROWNING CBN2202SB3APNM HELICAL GEAR REDUCER 1790IN-LBS 6.3:1 D593552
BROWNING CBN2202SB3APNM IN-LINE HELICAL GEAR REDUCER 2080IN-LBS 8:1 D593551
FALK 2107J14 QUADRIVE SHAFT MOUNT ENCLOSED GEAR DRIVE REDUCER 14.43:1 D596293
HANSEN JCF21C80I JUNIOR 80:1 HELICAL GEAR REDUCER D596376
DODGE 20QZ30R56 TIGEAR 2 0.96HP 30:1 GEAR REDUCER D570646
STOBER DRIVES K102AF0040MT10 SERVOFIT PRECISION GEARHEAD 4:1 D570884
CAMCO 250P-8H20-180 PARALLEL INDEX DRIVE D572292
HUB CITY 0220-68462-3204 500:1 WORM GEAR REDUCER D585497
NABTESCO RV-40E-153 P35E013C00 GEAR REDUCER D586107
HUB CITY 0220-68785-3206 WORM GEAR REDUCER 200:1 56C 5/8 X 1-7/16IN D588055
NUTTALL R32D 10PG74106 17.2HP 14.35:1 GEAR REDUCER D538339
BROWNING CBN3592BSV145U1?84TC 1-1/8 IN 2-3/8 IN 45:1 HELICAL GEAR REDUCER D546689
APEX ABR090-S2-P2 RIGHT ANGLE 25:1 GEAR REDUCER D570668
WITTENSTEIN ALPHA LP 155-MO2-25-111-?000 25:1 PLANETARY GEAR REDUCER D567065
GROVE GEAR GRG-HMQ-821-20-?H1-56-19 IRONMAN WORM GEAR REDUCER 56 20:1 D585112
CAMCO 387P2H28-180 MSHV55741-9C INDEX 50:1 GEAR REDUCER D572543
PARKER PX90-020-F01S2 20:1 PLANETARY GEARHEAD GEAR REDUCER D570685
WITTENSTEIN ALPHA SP 140S-MF2-50-1E1?-2S-P&G 50:1 HELICAL REDUCER D530776
NORD GEAR 52 VL IEC132 14:1 HELICAL GEAR REDUCER D562570
GAM GEAR DS-HB-115-120H-?090-1011-S086 DYNA 120:1 WORM GEAR REDUCER D560570
SEW EURODRIVE K77AM184 WORM GEAR REDUCER 30.89:1 D568824
DODGE HB682CN180TC QUANTIS 9.56HP 17.82:1 HELICAL GEAR REDUCER D546632
PATERSON NC529KE1UC00N01 HELICAL GEAR REDUCER 2-1/2HP 13.99:1 D581196
SEW EURODRIVE SAF67R37AM56 WORM GEAR REDUCER 712:1 D569778
STOBER K102AF0056MR140?/050 RIGHT ANGLE HELICAL/BEVEL GEAR REDUCER D570949
MASTER 7880206001DT 6.9HP 20.9:1 GEAR REDUCER D572100
BOSTON GEAR 732A 50F-G 1.73HP 50:1 GEAR REDUCER D572321
BAYSIDE RT142-009-002 9:1 PLANETARY GEARHEAD GEAR REDUCER D562831
STOBER DRIVES K202WG0460MT10 SERVOFIT 46.2:1 PRECISION GEARHEAD D562847
STOBER DRIVES K402AG0280MR160?/140 27.8:1 WORM GEAR REDUCER D562853
LENZE GST04-2N VBR 5B HELICAL GEAR REDUCER 19.36:1 D570771
BROWNING 932624 REV 1 RIGHT-ANGLE BEVEL GEAR REDUCER 1:1 5/8IN X 5/8IN D587282
WINSMITH 920MDNE SE WORM GEAR REDUCER 1750RPM 0.43HP 50:1 5/8IN 1IN D584995
BOSTON GEAR F231D-14-B9 REDUCTOR 14:1 HELICAL GEAR REDUCER D587029
CONE DRIVE S03010.SWSAD03B?CHCG 10:1 WORM GEAR REDUCER D567236
DODGE 26QZ40L56 TIGEAR 2 WORM GEAR REDUCER 1.55HP 40:1 D570781
CONE DRIVE S030SWSAD03 10:1 WORM GEAR REDUCER D567231
DAVID BROWN M0620BRN RADICON 3.6:1 HELICAL GEAR REDUCER D587042
ALPHA SP 075-MF1-7-131-0?00 7:1 PLANETARY GEARHEAD GEAR REDUCER D551927
ALPHA SP 075G-MF1-5-1E1-?2S-PGG 5:1 PLANETARY GEARHEAD GEAR REDUCER D551919
ALPHA TP 300-MF2-31-011-?000 31:1 PLANETARY GEARHEAD GEAR REDUCER D551929
LENZE GFL07-2W HBR 1F 27.119:1 WORM GEAR REDUCER D552176
STOBER DRIVES K403AG0670MR160?/140 2.1HP 67.3:1 HELICAL BEVEL REDUCER D562714
STOBER DRIVES K403WG1340MR160?/140B 134.4:1 WORM GEAR REDUCER D562849
NORD GEAR SK4382VZ/VL F 118.38:1 GEAR REDUCER D563219
DODGE 26Q60LR56 TIGEAR 2 WORM REDUCER 60:1 56C 1.08HP 5/8IN X 1-1/8IN D592973
BOSTON GEAR 338-50-G1 REDUCTOR RATIOMOTOR WORM GEAR REDUCER 2.95HP 50:1 D595047
FALK 1262WBM3A OMNIBOX WORM GEAR REDUCER 2.92HP 15:1 D596287
DAVID BROWN M0620BANR143TC RADICON 143TC 2.8:1 HELICAL GEAR REDUCER D587040
BROWNING 206ULR10 WORM GEAR REDUCER 1.95HP 10.1:1 D590961
BOSTON GEAR RF721-10-B5-G 700 2.14HP 10:1 WORM REDUCER D532572
WITTENSTEIN ALPHA LP120-MO1-5-111?-000 5:1 PLANETARY GEARHEAD D567196
WITTENSTEIN ALPHA LP120-MO1-10-11?1-000 10:1 PLANETARY GEARHEAD D567192
WITTENSTEIN ALPHA LP120-MO1-10-11?1-000 10:1 PLANETARY GEAR REDUCER D567073
DODGE 17QZ30R56 TIGEAR2 0.76HP 30:1 56 GEAR REDUCER D570954
BOSTON GEAR HF72660KPVB5HP2?0 0.89HP 60:1 WORM GEAR REDUCER D586074
WINSMITH E17MDNS175X0FA SE ENCORE WORM GEAR REDUCER 40:1 56C 0.39HP D590615
WITTENSTEIN ALPHA LP120-MO1-10-11?1-000 10:1 PLANETARY GEARHEAD D567194
SEW EURODRIVE S67AM56 29.63:1 WORM GEAR REDUCER D587034
ALPHA SP 075S-MC1-10-1E1?-2K-PS1 10:1 PLANETARY GEARHEAD GEAR REDUCER D551999
ALPHA SP 100S-MC1-5-1K1-?2K-PS1 5:1 PLANETARY GEARHEAD GEAR REDUCER D552000
FALK 50BU1A 70:1 WORM GEAR REDUCER D554013
GROVE GEAR GR-UMQ-824-15-L?-140 IRONMAN WORM GEAR REDUCER 2.765HP 15:1 D582684
MORSE ML0494 25GCT25 L 145T GEAR REDUCER 1.54HP 25:1 D572297
DODGE 140-TM3A-47.1-A?1 1.9HP 47.1:1 IN-LINE GEAR REDUCER D562739
STOBER K302WG0125MR160?/140B 12.6:1 5.6HP HELICAL BEVEL REDUCER D562740
HUB CITY 261 WORM GEAR REDUCER 5:1 3/4IN X 1-1/4IN D585616
WINSMITH 930CDSFE 3.01HP 20:1 WORM GEAR REDUCER D567220
WINSMITH 930CDSFE 2.14HP 30:1 WORM GEAR REDUCER D567216
WINSMITH 930CDSFE 2.14HP 30:1 WORM GEAR REDUCER D567223
GROVE GEAR BM0226-R2 FLEXALINE REDUCER 140TC 30:1 7/8IN X 1-1/8IN D585591
NORD 42 N250TC UNICASE REDUCER 116RPM 11009LB-IN 1-7/8IN 15.12:1 D588050
GROVE GEAR GR-DMQ-818-1200?-R5-56 IRONMAN 0.059HP 1200:1 WORM REDUCER D572792
MORSE 25GCT R 145T 0.94HP 25:1 145T WORM GEAR REDUCER D570869
MAREL HG 60E NA184/DQS-145 GEAR REDUCER 2.33:1 D570799
BONFIGLIOLI 300L220.1PCV01B F 20.1:1 IN-LINE PLANETARY GEAR REDUCER D549880
HUB CITY 0221-09988-596 150 1:1 BEVEL GEAR REDUCER D584382
GROVE GEAR H226-1A FLEXALINE 10:1 WORM GEAR REDUCER D577406
SEW EURODRIVE SA57/TAM145 WORM GEAR REDUCER 94.08:1 D571383
GROVE GEAR GRG-TM821-00-L-?56-NL 60:1 WORM GEAR REDUCER D577440
HUB CITY 261 5/8IN 1-1/4IN 5:1 WORM GEAR REDUCER D585609
ORIENTAL MOTOR 4GN36K GEARHEAD PARALLEL SHAFT 36:1 D591231
CONE DRIVE HU76-4 GEAR REDUCER 10:1 2.98KW D562331
WITTENSTEIN ALPHA HG 075F-MF2-25-5E1?-1S10 25:1 RIGHT ANGLE GEARHEAD D562226
WINSMITH 930CDNE32000DN 3.01HP 145TC 20:1 WORM GEAR REDUCER D562702
STOBER DRIVES C302N0140MR165/?050 6.3HP 14:1 HELICAL REDUCER D562806
LEESON DUMQ826-R2-56C WTXM01-7.5-56 IRONMAN GEAR REDUCER ASSEMBLY D562798
GAM GEAR 700599 EPL-W-084-005G-?115-B05-S558 5:1 PLANETARY GEARHEAD D562793
ELECTROPOWER FSR 3 MFT 15.35:1 HELICAL GEAR REDUCER D561218
FALK 03UCBN2A1.8A A ULTRAMITE HELICAL CONCENTRIC GEAR REDUCER 56C 1.94:1 D592344
DODGE SR96022L1 S175B020N000L1 TIGEAR WORM GEAR REDUCER 1HP 20:1 D593487
HUB CITY 0220-23237-385 WORM GEAR REDUCER 30:1 D570797
DODGE S175B010N000K1 TIGEAR WORM GEAR REDUCER 1.70HP 10:1 D595875
DORRIS 1817LH SGC 16.8:1 GEAR DRIVE REDUCER D540284
ALPHA LPB 090-M01-10-110-?PGG 10:1 PLANETARY GEARHEAD GEAR REDUCER D551926
DODGE 202Q20R56 TIGEAR2 20:1 WORM GEAR REDUCER D584781
HUB CITY 212 0220-61013-212 WORM GEAR REDUCER 20:1 D585434
GROVE GEAR HMQ226-1 FLEXALINE WORM GEAR REDUCER 56C 0.386HP 100:1 D587331
BOSTON GEAR SRF721V-5-B5-G 700 3.66HP 5:1 WORM REDUCER D532573
STOBER K102WG0140MR160?/140B 14.1:1 RIGHT ANGLE HELICAL BEVEL REDUCER D562701
RELEX AG KP 40A-12/M2/LS/OP?2 PLANETARY GEARHEAD D562781
STOBER K102WG0140MR160?/050B 14.1:1 RIGHT ANGLE HELICAL BEVEL REDUCER D562705
STOBER DRIVES K302WG0230MR160?/140B 23.3:1 WORM GEAR REDUCER D560577
SEW EURODRIVE R57AD2 30.18:1 WORM GEAR REDUCER D560614
SEW EURODRIVE S67A-KS 41.89:1 WORM GEAR REDUCER D562315
MORSE 25GCT WORM GEAR REDUCER 56C 2.37HP 15:1 D594341
EUCLID 300A-B1963C 30:1 RIGHT ANGLE WORM GEAR REDUCER D585858
RELIANCE M641949001QT 56CM12A MASTER XL 162:1 WORM GEAR REDUCER D565814
WINSMITH 930CDSN 2.14HP 30:1 WORM GEAR REDUCER D567207
DODGE 15QZ15L56WP TIGEAR 2 0.86HP 15:1 GEAR REDUCER D570676
WINSMITH E20 XDTS SE ENCORE 1.24HP 20:1 GEAR REDUCER D570660
HUB CITY 0220-00814-150 1:1 BEVEL GEAR REDUCER D570682
WINSMITH E20 XDTS SE ENCORE 2.24HP 20:1 GEAR REDUCER D571164
SEW EURODRIVE KT37AQA115/3 29.96:1 GEAR REDUCER D571283
WINSMITH 4.5DBI PARALLEL GEAR REDUCER 1.14HP 120:1 D567756
ALPHA SP 100-MF1-5-141-P?&G 5:1 PLANETARY GEARHEAD GEAR REDUCER D551966
BROWNING HWN3152-S2-B33D?-V-143 12.2:1 HELICAL WORM GEAR REDUCER D562335
NEUGART PLN 90 PLANETARY GEARHEAD 40:1 D562736
TYPE E32 900NCM 15:1 GEAR REDUCER D563056
STOBER DRIVES K302WG0100MR160?/140B 10.1:1 WORM GEAR REDUCER D560576
US MOTORS CBN1702SV6U56NM SERIES 2000 HELICAL GEAR REDUCER 9:1 D592850
MORSE G09K XX5011 60:1 RH WORM GEAR REDUCER D532353
WINSMITH E24 MSFS SE ENCORE 3.67HP 10:1 56C WORM GEAR REDUCER D555584
ALPHA SP 060-MF1-5-131-P?&G 5:1 PLANETARY GEARHEAD GEAR REDUCER D552133
SEW EURODRIVE SA42LP56C 58.50:1 WORM GEAR REDUCER D584060
GROVE GEAR BFMCQ12-G FLEX-IN-LINE GEAR REDUCER 12.5:1 56C 1.08HP D593537
HUB CITY 264 0220-62058 25:1 WORM GEAR REDUCER D585354
HUB CITY 264 0220-77832 WORM GEAR REDUCER 25:1 5/8IN X 1-1/4IN D585832
DODGE 23QZ15R56WP TIGEAR 2 2.57HP 15:1 WORM GEAR REDUCER D570664
DODGE 17QZ20R14 TIGEAR2 WORM GEAR REDUCER 1.03HP 20:1 D570861
DODGE 17QZ50R56 TIGEAR2 0.51HP 50:1 GEAR REDUCER D570864
HUB CITY 0221-16716-66M 2:1 BEVEL GEAR REDUCER D562699
DAVID BROWN M042250 BANRI 50:1 GEAR REDUCER D564501
TEXTRON B03-57524 CONE DRIVE 15:1 WORM GEAR REDUCER D582638
DODGE M60473 140DM21A17 MASTER XL 4.10HP 17.1:1 HELICAL GEAR REDUCER D548098
VIKING 3-551-012-795-F?C 7.95:1 HELICAL GEAR REDUCER D556285
ZAE KRS 063 535.91:1 GEAR REDUCER D578891
ZAE KRS 063 461386-0046 535.91:1 GEAR REDUCER D578886
ZAE KRS 063 535.91:1 GEAR REDUCER D578887
DODGE 26S40R TIGEAR 1.67HP 40:1 WORM GEAR REDUCER D549532
DODGE 56-TM2A-70.6-A1 0.6HP 70.6:1 HELICAL GEAR REDUCER D548057
SERVOMECH 01.063.760/2341?.2 1:70 GEAR REDUCER D549749
MORSE 25GCT 0.64HP 60:1 WORM GEAR REDUCER D559617
BOSTON GEAR SF718V60SB5J 0.32HP 60:1 WORM GEAR REDUCER D562800
SEW EURODRIVE FA37AQA115/3 8.97:1 WORM GEAR REDUCER D562870
GROVE GEAR TXQ3 210/180 FLEX-IN-LINE 10HP 5:1 HELICAL GEAR REDUCER D562941
GROVE GEAR BFMCQ12-G FLEX-IN-LINE 56C 0.91HP 16:1 GEAR REDUCER D582990
GROVE GEAR BFMCQ12-G-I FLEX-IN-LINE GEAR REDUCER 20.9:1 56C 0.73HP D593589
ORIENTAL MOTOR 5GU60RA PLANETARY GEARHEAD 5/8IN X 5/8IN D593945
DODGE MP94611L1 M SZ Q200B005M140L1 TIGEAR WORM GEAR REDUCER 3.20HP 5:1 D594360
MOTOVARIO NRV-50 5:1 WORM GEAR REDUCER D595045
US MOTORS L04-864517-011 SERIES 3000 HELICAL GEAR REDUCER 7.1:1 D595646