Mitsubishi Electronics FX1S manual How to Read Ladder Logic, Example

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FX Series Programmable Controllers

Basic Program Instructions 2

2.3How to Read Ladder Logic

Ladder logic is very closely associated to basic relay logic. There are both contacts and coils that can be loaded and driven in different configurations. However, the basic principle remains the same.

A coil drives direct outputs of the PLC (ex. a Y device) or drives internal timers, counters or flags (ex. T, C, M and S devices). Each coil has associated contacts. These contacts are available in both “normally open” (NO) and “normally closed” (NC) configurations.

The term “normal(ly)” refers to the status of the contacts when the coil is not energized. Using a relay analogy, when the coil is OFF, a NO contact would have no current flow, that is, a load being supplied through a NO contact would not operate. However, a NC contact would allow current to flow, hence the connected load would be active.

Activating the coil reverses the contact status, that is, the current would flow in a NO contact and a NC contact would inhibit the flow.

Physical inputs to the PLC (X devices) have no programmable coil. These devices may only be used in a contact format (NO and NC types are available).

Example:

Because of the close relay association, ladder logic programs can be read as current flowing from the left vertical line to the right vertical line. This current must pass through a series of contact representations such as X0 and X1 in order to switch the output coil Y0 ON. Therefore, in the example shown, switching X0 ON causes the output Y0 to also switch ON. If however, the limit switch X1 is activates, the output Y0 turns OFF. This is because the connection between the left and the right vertical lines breaks so there is no current flow.

						Motor
Toggle switch		Programmable Controller
		Programmable Controller
X0				O	Y0
X0	I	PC Program		O
	I			U
	N

				T
	P	X0	X1	T
	P	X0	X1	P
	U			P		AC
	U		Y0	U	COM
X1	T		Y0	U
X1	T			T		Power
				T	(Y0)	Supply
					(Y0)	Supply
Limit switch
		DC Power Supply

2-2

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Contents Programming Manual Page FX Series Programmable Controllers ForewordFX Series Programmable Controllers FAX Back Combined Programming Manual J FX Series Programmable Controllers Hardware Warnings Software WarningsFX Series Programmable Controllers Contents STL Programming Applied Instructions Rotation And Shift Functions 30 to External FX Serial Devices Functions 80 to Execution Times And Instructional 10-1 Viii FX Series Programmable Controllers Chapter Contents Introduction OverviewWhat is a Programmable Controller? What do You Need to Program a PLC?Special considerations for programming equipment Current Generation CPU all versionsAssocciated Manuals Manual name Number FX Base Unit HardwareManual name Number FX DU, GOT and DM units Memo Basic Program Instructions FX Series Programmable ControllersBasic Program Instructions Outline of Basic Devices Used in Programming What is a Program?Detailed device information How to Read Ladder Logic ExampleOUT instruction Load, Load InverseProgram example Timer and Counter Variations OutLast coil effect Use of dual coilsDouble Coil Designation And, And Inverse Peripheral limitationsOr, Or Inverse ORILoad Pulse, Load Trailing Pulse Single Operation flags M2800 to M3071Pulse, And Trailing Pulse LDF ANF OUTOr Pulse, Or Trailing Pulse ORF ORBBatch processing limitations Or BlockSequential processing limitations Block ANB13 MPS, MRD and MPP MPS, MRD and MPP usageMultiple program examples Master Control and Reset MCRNested MC program example Set and Reset Resetting timers and countersRetentive timers Timer, Counter Out & Reset16.1Basic Timers, Retentive Timers And Counters Bit counters Availability of devicesNormal 32 bit Counters High Speed CountersPLF Leading and Trailing PulseInverse Usages for INVNo Operation No Operation20 End Program scanMemo STL Programming FX Series Programmable ControllersSTL Programming What is STL, SFC And IEC1131 Part 3? General noteEach step is a program How STL OperatesLook Inside an STL How To Start And End An STL Program Combined SFC Ladder representationEmbedded STL programs Activating new statesTerminating an STL Program Initial StepsReturning to Standard Ladder Using SET to drive an STL coil Moving Between STL StepsOUT is used for loops and jumps Using OUT to drive an STL coilOut is used for distant jumps Rules and Techniques For STL programs Basic Notes On The Behavior Of STL programsT001 K20 K50 Method 1 Using locking devices Single Signal Step ControlMethod 2 Special Single Pulse Flags Using ‘jump’ operations with STL Restrictions Of Some Instructions When Used With STLRestrictions on using applied instructions Using STL To Select The Most Appropriate Program STL OUT SETUsing STL To Activate Multiple Flows Simultaneously Limits on the number of branchesLimits on the number of branches General Rules For Successful STL Branching Instruction FormatGeneral Precautions When Using The FX-PCS/AT-EE Software Programming Examples Simple STL FlowIdentification of normally closed contacts SET STLPoints to note Selective Branch/ First State Merge Example ProgramFull STL flow diagram/program Advanced STL Use Devices in Detail FX Series Programmable ControllersDevices in Detail Configuration details InputsAvailable devices Device MnemonicDevice Mnemonic Y OutputsAlias O/P Device Mnemonic M Auxiliary RelaysGeneral Stable State Auxiliary Relays Battery Backed/ Latched Auxiliary Relays External loadsSpecial Diagnostic Auxiliary Relays Special Single Operation Pulse RelaysDevice Mnemonic S State RelaysGeneral Stable State State Relays Battery Backed/ Latched State Relays PLC FX 1S FX 1N FX 2NAssigned states Monitoring STL programsSTL/SFC programming IST instructionAnnunciator Flags Pointers Jumping to the end of the programDevice availability Device Mnemonic PInterrupt Pointers Additional applied instructionsNested levels Pointer positionRules of use Timer InterruptsInput Interrupts Driving special auxiliary relays Disabling high speed counter interruptsAdditional notes Disabling Individual InterruptsConstant K Constant HDevice Mnemonic K Example device usage N/ATimer accuracy TimersDevice Mnemonic T Selectable Timers General timer operationDriving special auxiliary coils Using timers in interrupt or ‘CALL’ subroutines Retentive TimersInternal timer accuracy Timers Used in Interrupt and ‘CALL’ SubroutinesTimer Accuracy ConditionSetting ranges for counters CountersHigh speed counters Device Mnemonic CBattery backed/latched counters General/ Latched 16bit UP CountersSelecting the counting direction Battery backed/ latched countersGeneral/ Latched 32bit Bi-directional Counters Basic high speed counter operation Further uses NoneDriving high speed counter coils Basic High Speed Counter OperationCounter Speeds Input assignmentAvailability of High Speed Counters Calculating the maximum combined counting speed on FX1S Device specification Setting rangeDirection setting Using the SPD instructionRST 11.5 2 Phase Bi-directional Counters C246 to C250 Device size11.6 A/B Phase Counters C252 to C255 Device Mnemonic D Data RegistersExample device usage None Data retention Data register updatesGeneral Use Registers Using the FX2-40AW/AP Use of diagnostic registersBattery Backed/ Latched Registers Special Diagnostic RegistersSpecial caution when using FX1S Writing to file registersFile Registers Program memory registersExternally Adjusted Registers UsesIndex Registers Use of Modifiers with Applied Instruction ParametersDevice Mnemonic V,Z Available formsModifying a Constant Using Multiple Index RegistersMisuse of the Modifiers Bits, Words, BCD and Hexadecimal Bit Devices, Individual and GroupedAssigning I/O Moving grouped bit devicesAssigning grouped bit devices Word Devices Interpreting Word DataFX Series Programmable Controllers Binary Coded Decimal value= Error Word Data Summary14.4 Two’s Compliment Inverted7 Additional1Floating Point And Scientific Notation Some useful constantsScientific Notation Floating Point Format FLT