m340 plc和编码器串行rs485通信采用modbus rtu通信规约，这个通讯协议已广泛被国内外各行业作为系统集成的一种通用工业标准协议，有利于系统的维护和扩展。plc为主站，编码器为从站。

　　查编码器技术手册，水位测值的modbus地址是4x0000，根据modbus通信规约信息帧结构读取地址 1 传感器的水位测量值，应发送以下通信码：m340 plc读取水位信息主要用read_var功能模块：140CRA21110分布式I/O  

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　　01 03 0000 0001 840a

　　站地址 　功能码 　　首地址 　　个数　　 crc校验码

　　功能模块说明[1]

　　adr

　　通信地址：语法为 addm (`r.m.c.node`机架号.模块号.通道号.站地址) 类型。

　　obj 要读取的对象类型

　　’%m’：内部位

　　’%mw’：内部字

　　’%s’：系统位

　　’%sw’：系统字

　　num 读取的个对象的索引。

　　nb 要读取的对象的数量。

　　recp输出参数包含所读取对象的值的字表。

　　gest交换管理表：4个字的数组。

　　表1 交换管理表

Omega FMA5416ST Gas Mass Flow Controller, Gas: N2, Max Gas Pressure: 500psi

　　Allen Bradley 800T-D Mushroom Head E-Stop Button w/800T-XA Contact Block 1NO/1NC

　　Lot of 3 Allen Bradley 800T-PA16G Momentary Lighted Green Push Button 1NO/1NC

　　Banner S18SN6FF100Q Photoelectric Sensor,10-30VDC, Sensing Range: 100mm

　　Festo MFH-5-1/8 Pneumatic Valve, 5-Port 2-Position, 24VDC-42VAC, Ports: G 1/8

　　Lot of 2 New Sloan F5A CP Royal Spud Coupling Assembly 1 1/4" x 3" Chrome Finish

　　Cutler Hammer C400LG1 Start/Stop Push Button Switch 1N.C. & 1N.O.

　　Lot of 3 New Sloan F5AW Flushometer Spud Coupling & Flange Kit for 3/4" Urinal

　　New Telemecanique LT2-SA 00B Thermal Trip Protection Relay 24VAC

　　Turck BI1,5U-EG08-AN6X-V1131 Inductive Prox. Sensor, 10-30VDC 150mA, 3-Wire NPN

　　Hoffman CSD242010 Electrical Enclosure, Dimensions: 24" x 20" x 10", *Holes*

　　Omron ZE-NA2-2S Limit Switch 15A 480VAC 250VDC Max

　　Idec AVLW49902D-R-24V E-Stop Pushbutton 110-600VAC 120-600VDC 10A 1NO 2NC

　　Hoffman CSD12126 Electrical Enclosure, Dimensions: 12" x 12" x 6", *Holes*

　　THK HRW21CA Carriage on 780mm Long Rail, Dimensions: 21mm x 68mm x 58.8mm

　　Lot of 2 IDEC RH2B-U DC24V Relays 24VDC Coil 120/240VAC 10A Contacts DPDT

　　THK HRW21CA Carriage on 380mm Long Rail, Dimensions: 21mm x 68mm x 58.8mm

　　Telemecanique LC1 D2510 Contactor 3-Pole + N.O Aux Rated @ 40A 1000V, 120V Coil

　　Lot of 2 IDEC RH4B-U DC24V Relays 24VDC Coil 120/240VAC 7.5/5A Contacts 4PDT

　　New ACE MC75-3 Miniature Shock Absorber 75in-lbs/cycle 0.4" Stroke 1/2-20 Thread

　　SMC MGPL32-300 Guide Cylinder, Bore: 32mm, Stroke: 300mm, Ports: 1/8 RC

　　Allen Bradley 100-C16*10 Contactor 3-Pole + N.O. Rated 32A 690V, 110/120V Coil

　　Allen Bradley 100-C09*10 Contactor 3-Pole + N.O. Rated 32A 690V, 110/120V Coil

　　Parker RS33C-DNNPS Stepper Motor, Shaft: 0.375" Diameter, 340VDC Winding

　　Allen Bradley 100-C23*10 Contactor 3-Pole + N.O. Rated 32A 690V, 110/120V Coil

　　Manifold of 4 SMC SY3140-5LOZ Solenoid Valves With Interface Regulators & Gauges

　　Lot of 3 New ILSCO PB2-300 Aluminum Panelboard Lugs

　　New SMC RSDQB20-10DR-F79SAPC Stopper Cylinder w/ Roller Rod End 10mm Stroke 1MPa

　　Parker Compumotor Zeta4-240 Indexer / Stepper Motor w/ Power Cable

　　Contemporary Control Systems EISK8-100T Ethernet Module 8 Port 10-36VDC 24VAC

　　Watlow 96B1-CDDM-00RR Temperature Controller 24-28VAC/DC 7VA w/ Bracket

　　Horner Electric HE800STP100H Stepper Control Module for OSC Smart Stack

　　Lot of 3 New ADR25-21 2-Conductor Lug

　　GE IC300ADC110B Analog Input Smart Stack Module 4 Channel 12-Bit Resolution

　　GE Fanuc IC3000CS200H Operator Interface Panel 24VDC 180mAABB A8PU05ABFW02

　　ABB 3HNM09846-1

　　ABB 3HAC14551-2

　　ABB 3HAC14550-4

　　ABB 3HAC14549-3/09A

　　ABB 3AUA489002B4562

　　ABB 35AE92A

　　ABB 2012AZ10101B

　　ABB 1SAR330020R0000

　　ABB 07SK90R1

　　ABB 07MK92

　　ABB 07KT94

　　ABB 07KT93

　　ABB 07KT92

　　ABB 07KP90

　　ABB 07DI92

　　ABB 07DC92

　　ABB 07DC91

　　ABB 07CR41

　　ABB 07AI91

　　ABB 07AI90-S

　　ABB 07AC91

　　SMC CXSM10-60 Dual Rod Cylinder, 0.7MPa Max (101psi), 10mm Bore, 60mm Stroke

　　G-Dok GD-80 Footmaster Swivel Caster with Leveling Leg 500kg Capacity per Caster

　　Hose Master Stainless Corrugated Hose With Stainless Steel Braid, 36" Long

　　Cutler Hammer C10CN3 Contactor 3-Pole Rated @ 575V 27A, 220/240VAC Coil, + Aux

　　Flex Cable FC-CFBM6DF-CBAF-M003 Feedback Cable, 28-Pin to Flying Leads, 3m Long

　　Flex-Cable FC-FPBM6DF-16AF-M003 Motor Power Cable w/ Brake Flying Lead

　　New In Box Pentair SF0524414 Filter Fan, Power: 24VDC 0.17A, Speed: 3050RPM