How to Read Ladder Logic, Example | Mitsubishi Electronics FX1S

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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Mitsubishi Electronics FX1S manual How to Read Ladder Logic, Example

Contents

Programming Manual Page FX Series Programmable Controllers Foreword FX Series Programmable Controllers FAX Back Combined Programming Manual J FX Series Programmable Controllers Hardware Warnings Software Warnings FX 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 Overview What is a Programmable Controller? What do You Need to Program a PLC? Special considerations for programming equipment Current Generation CPU all versions Assocciated Manuals Manual name Number FX Base Unit Hardware Manual 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 Example OUT instruction Load, Load InverseProgram example Timer and Counter Variations Out Last coil effect Use of dual coilsDouble Coil Designation And, And Inverse Peripheral limitations Or, Or Inverse ORI Load Pulse, Load Trailing Pulse Single Operation flags M2800 to M3071 Pulse, And Trailing Pulse LDF ANF OUT Or Pulse, Or Trailing Pulse ORF ORB Batch processing limitations Or BlockSequential processing limitations Block ANB 13 MPS, MRD and MPP MPS, MRD and MPP usage Multiple program examples Master Control and Reset MCR Nested MC program example Set and Reset Resetting timers and counters Retentive timers Timer, Counter Out & Reset16.1Basic Timers, Retentive Timers And Counters Bit counters Availability of devicesNormal 32 bit Counters High Speed Counters PLF Leading and Trailing Pulse Inverse Usages for INV No Operation No Operation 20 End Program scan Memo STL Programming FX Series Programmable ControllersSTL Programming What is STL, SFC And IEC1131 Part 3? General note Each 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 states Terminating an STL Program Initial StepsReturning to Standard Ladder Using SET to drive an STL coil Moving Between STL Steps OUT 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 programs T001 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 SET Using STL To Activate Multiple Flows Simultaneously Limits on the number of branches Limits on the number of branches General Rules For Successful STL Branching Instruction Format General Precautions When Using The FX-PCS/AT-EE Software Programming Examples Simple STL Flow Identification of normally closed contacts SET STL Points to note Selective Branch/ First State Merge Example Program Full STL flow diagram/program Advanced STL Use Devices in Detail FX Series Programmable ControllersDevices in Detail Configuration details InputsAvailable devices Device Mnemonic Device Mnemonic Y OutputsAlias O/P Device Mnemonic M Auxiliary RelaysGeneral Stable State Auxiliary Relays Battery Backed/ Latched Auxiliary Relays External loads Special Diagnostic Auxiliary Relays Special Single Operation Pulse Relays Device Mnemonic S State RelaysGeneral Stable State State Relays Battery Backed/ Latched State Relays PLC FX 1S FX 1N FX 2N Assigned states Monitoring STL programsSTL/SFC programming IST instruction Annunciator Flags Pointers Jumping to the end of the programDevice availability Device Mnemonic P Interrupt Pointers Additional applied instructionsNested levels Pointer position Rules of use Timer InterruptsInput Interrupts Driving special auxiliary relays Disabling high speed counter interruptsAdditional notes Disabling Individual Interrupts Constant K Constant HDevice Mnemonic K Example device usage N/A Timer accuracy TimersDevice Mnemonic T Selectable Timers General timer operationDriving special auxiliary coils Using timers in interrupt or ‘CALL’ subroutines Retentive Timers Internal timer accuracy Timers Used in Interrupt and ‘CALL’ SubroutinesTimer Accuracy Condition Setting ranges for counters CountersHigh speed counters Device Mnemonic C Battery backed/latched counters General/ Latched 16bit UP Counters Selecting the counting direction Battery backed/ latched countersGeneral/ Latched 32bit Bi-directional Counters Basic high speed counter operation Further uses None Driving high speed counter coils Basic High Speed Counter Operation Counter Speeds Input assignmentAvailability of High Speed Counters Calculating the maximum combined counting speed on FX1S Device specification Setting rangeDirection setting Using the SPD instruction RST 11.5 2 Phase Bi-directional Counters C246 to C250 Device size 11.6 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 Registers Special caution when using FX1S Writing to file registersFile Registers Program memory registers Externally Adjusted Registers Uses Index Registers Use of Modifiers with Applied Instruction ParametersDevice Mnemonic V,Z Available forms Modifying a Constant Using Multiple Index RegistersMisuse of the Modifiers Bits, Words, BCD and Hexadecimal Bit Devices, Individual and Grouped Assigning I/O Moving grouped bit devicesAssigning grouped bit devices Word Devices Interpreting Word Data FX Series Programmable Controllers Binary Coded Decimal value= Error Word Data Summary 14.4 Two’s Compliment Inverted7 Additional1 Floating Point And Scientific Notation Some useful constants Scientific Notation Floating Point Format FLT