Simple Ladder Logic. Simple Ladder Logic. OR Operation. Chapter 2: Basic Ladder Logic Programming. Ladder Logic Learning objectives.

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1 OR Operation Chapter 2: asic Ladder Logic Programming Control ehavior: The light should be on when either switch is on (i.e., closed) or switch is on (closed). Otherwise it should be off Task: Implement this behavior using Relay circuit puter ided Manufacturing TCH 4/5335 puter ided Manufacturing TCH 4/ Ladder Logic Learning objectives Understand basic ladder logic symbol OR Operation Possible binations of the 2 Switches: (2 2 ) OR Truth Table Write ladder logic for simple applications Translate relay ladder logic into PLC ladder logic OFF OFF OFF OFF ON ON ON OFF ON ON ON ON puter ided Manufacturing TCH 4/ puter ided Manufacturing TCH 4/ Ladder Logic: Primary Programming Language for PLCs. Visual and Graphical language unlike textual high-level, such as C, C++, Java erived from relay logic diagrams Primitive Logic Operations: OR N NOT OR Operation Relay Circuit Switches and are connected in parallel to relay coils R & R resp. R R When switch (or switch ) is closed relay coil R (or R) gets energized L The Normally Open (NO) contact R (or R) gets closed Power is transmitted to coil Relay coil gets energized The NO contact gets closed Power is transmitted to the bulb puter ided Manufacturing TCH 4/ puter ided Manufacturing TCH 4/5335 6

2 OR Operation Relay Ladder Logic Circuit N Operation 2v R R Neutral Possible binations of the 2 Switches: (2 2 ) N Truth Table OFF OFF OFF R OFF ON OFF ON OFF OFF R ON ON ON Lamp W puter ided Manufacturing TCH 4/ puter ided Manufacturing TCH 4/5335 OR Operation N Operation Relay Circuit R R L ppend above to the leading two rungs of relay ladder logic diagram Switches and are connected in series to relay coils R & R resp. When switch is closed relay coil R gets energized Switch and Switch are connected to discrete input channels of the PLC is connected to discrete output channel (actuator) of the PLC When input switch (or switch ) is on the light is on The Normally Open (NO) contact R gets closed Power flows to Normally Open (NO) contact R, where it terminates until R is energized Subsequently, when R gets energized, is energized, which causes the NO contact to close Power is transmitted to the bulb What happens if R is energized before R? puter ided Manufacturing TCH 4/ puter ided Manufacturing TCH 4/5335 N Operation Control ehavior: The light should be on when switch is on (i.e., closed) and switch is on (closed). Otherwise it should be off N Operation Circuit Task: Implement this behavior using Relay circuit puter ided Manufacturing TCH 4/ puter ided Manufacturing TCH 4/5335 2

3 NOT Operation NOT Operation Control ehavior: The light comes on only when switch is on (i.e., closed) and switch is off (open). Otherwise it should be off Task: Implement this behavior using Relay circuit puter ided Manufacturing TCH 4/ puter ided Manufacturing TCH 4/ NOT Operation NN Operation Possible binations of the 2 Switches: (2 2 ) NOT Truth Table OFF OFF OFF NN (NOT N) Control ehavior: The light comes on only when switch is off and switch is off. Otherwise it should be off OFF ON OFF ON OFF ON ON ON OFF Task: Implement this behavior using Relay circuit puter ided Manufacturing TCH 4/ puter ided Manufacturing TCH 4/ NOT Operation Relay Circuit NN Operation R R L Possible binations of the 2 Switches: (2 2 ) NN Truth Table OFF OFF ON Switches and are connected to relay coils R & R resp. When switch is closed relay coil R gets energized When switch is off (on) relay coil R is not energized (energized) and R contact is normally-closed (normally-open) OFF ON ON ON OFF ON ON ON OFF puter ided Manufacturing TCH 4/ puter ided Manufacturing TCH 4/5335 8

4 NN Operation Relay Circuit asic Ladder Logic Symbol Normally open contact Passes power (ON) if coil driving the contact is ON (closed) llen-radley calls it XIC - examine If Closed Normally closed contact Passes power (ON) if coil driving the contact is off (open) llen-radley calls it XIO - examine If Open or coil If any left-to-right path of inputs passes power, output is energized llen-radley calls it OT - OuTput nergize Not or coil If any left-to-right path of inputs passes power, output is de-energized The IC 63-3 standards describe the complete list of ladder logic contact and coil symbols. See also section 2.3. puter ided Manufacturing TCH 4/ puter ided Manufacturing TCH 4/ NN Operation Ladder Logic Circuit Symbols The symbols are ladder logic instructions The PLC scans (executes) the symbols: = on = Closed = True = = off = Open = False = very PLC manufacturer uses instruction symbols Industry trend is based on IC 63-3 Variations in symbols by Manufacturers llen-radley ControlLogix symbols slightly different (Refer 2.3.3) puter ided Manufacturing TCH 4/ puter ided Manufacturing TCH 4/ igital Logic Gates N OR NN + + Power Rails - Pair of Vertical Lines Rungs - Horizontal Lines Contacts,, C, arranged on rungs Note in : No Real Power Flow (like in relay ladder) There must be continuous path thru the contacts to energize the output Input (Condition) Instructions C F G H K Instruction Out Out2 NOR + + Continuous path for logic continuity Power flows NOT puter ided Manufacturing TCH 4/ puter ided Manufacturing TCH 4/

5 Instructions Two Classes of Ladder Logic Instructions Input (Condition) Instructions C Instruction Out xample SS2 P4 LS3 PL2 : ppears on extreme RHS of rung always Out, Out2 Input: ny instruction that can replace a contact F G H Out2 K Continuous path for logic continuity Power flows Can contacts appear on the RHS of a coil? puter ided Manufacturing TCH 4/ puter ided Manufacturing TCH 4/ Function lock Instructions xample 2 Task: Function lock Instructions ny non-contact instruction: Timer Instruction Counter Instruction parison Instruction H Input Function lock Function lock ( ) ( ) raw a ladder diagram that is equivalent to the following digital logic diagram C + Y Y is on when ( is on, is on and C is off ) or is on, or is off What is the oolean logic expression? puter ided Manufacturing TCH 4/ puter ided Manufacturing TCH 4/ xample Task: raw a ladder diagram that will cause the output, pilot light PL2, to be on when selector switch SS2 is closed, push button P4 is closed and limit switch LS3 is open. (Note: no I/O addresses yet.) Thought Process Identify the output: PL2 PL2 appears on rhs of rung What is the behavior (type of connection to use): sequential operation of all switches series connection Type of contacts to implement output: xample 2 Thought Process Identify the output: Y Coil Y appears on rhs of rung What is the behavior (type of connection to use): The inputs,, C for N gate will be connected in series The, inputs for OR gate will be connected in parallel with the output of N gate Type of contacts to implement output (review the expected behavior again to determine contact types): is on: is on: C is off: SS2 closed P4 closed LS3 open is on: is off: puter ided Manufacturing TCH 4/ puter ided Manufacturing TCH 4/5335 3

6 xample 2 angers Repeated - Correction C Y First consider the output Next, consider LL the conditions that drive the output (Out) (Implement the conditions in parallel) What happens if the contact refers to Y? puter ided Manufacturing TCH 4/ puter ided Manufacturing TCH 4/ Sealing an output angers C Y Y Use set/seal (latch) and reset (unlatch) together: If a set coil refers to an output there should be a reset coil for that output Reverse power flow in contact matrix is not allowed Power flow one way left to right (solid state relays) Y is set (latched) indefinitely puter ided Manufacturing TCH 4/ puter ided Manufacturing TCH 4/ angers Repeated o not repeat normal output coils that refer to the same address C Out angers Reverse Power Flow This is not allowed: L C F Reverse Power Flow PL L2 Other ladder logic F G H Out L C PL L2 K The coils for first and second rung refer to Out Second rung overrides the logic in first rung If the reverse power flow path is truly needed, then put it as a separate path, where the power flows from left to right: F F C puter ided Manufacturing TCH 4/ puter ided Manufacturing TCH 4/

7 Typical PLC Processor Scan Major tasks in a scan Read Inputs xecute Ladder Logic Update s Scan Time Processor must read the state of the physical inputs and set the state of the Physical outputs Typical PLC Processor Scan llen-radley RSLogix 5 The execution of PLC Processor controlled by processor mode (Refer to lab ) Run Mode: Physical Input, Physical s and Ladder logic all get scanned Remote Mode own load ladder logic to PLC Processor; and initiate scan from the remote terminal Program Mode Ladder logic not scanned puter ided Manufacturing TCH 4/ puter ided Manufacturing TCH 4/ Typical PLC Processor Scan Order of PLC Processor Scan Read Physical Inputs Scan ladder logic program Write the physical outputs Scan Time Time to complete above cycle Order of -2 milliseconds Update s Start Program (ladder logic) execution Read Inputs For most PLC's, the ladder scan starts at the top of the ladder and proceeds to the bottom of the ladder, examining each rung from left to right. Once a rung is examined, it is not examined again until the next scan. The rungs are not examined in reverse order. The JMP instruction may be used to jump back up the ladder and execute earlier rungs. Use of JMP not recommended Why? What could happen if scan time exceeds more than 2 milliseconds? puter ided Manufacturing TCH 4/ puter ided Manufacturing TCH 4/ Typical PLC Processor Scan Scenario 2 The state of actual input devices are copied to an area of the PLC Memory, input data table before the ladder logic program executes State of ctual Input evice Input Image (PLC Memory) Ladder Logic Program Runs s the ladder logic program is scanned, it reads the input data table then writes to a portion of PLC memory - the output data, table as it executes The output data table is copied to the actual output devices after the ladder logic has been scanned. Image (PLC Memory) What is the significance of the input and output data tables? State of ctual evice Push utton (P) Start P: PL4 PL3 P PL2 PL PL2 PL3 PL4 Scan 5 P PL PL2 PL3 PL4 Physical Input: P Physical s: PL, PL2, PL3 and PL4 Start of PLC scans State of PLC image memory for I/O devices: puter ided Manufacturing TCH 4/ puter ided Manufacturing TCH 4/

8 Push utton (P) Push utton (P) Scan : Only the state of P changes to ON () during the scan Scan 2: The ON state of P is copied into Input data table before Ladder logic is scanned Scans 5 and 6: Nothing Changes Scans 7 9 : Similar to Scans 2 4 except that state changes from (on) to (off) When rung is scanned PL is still off (o) When rung 2 is scanned PL2 is still off (o) Why? What is the value of PL4 and PL3 in ata table? When rung 3 is scanned the Value of PL3 in the output data table changes to Why? When rung 4 is scanned, the Value of PL4 in the output data table remains at off (). Why? t the end of scan 2 the values in data table are copied to the Physical evices. PL 3 turns on puter ided Manufacturing TCH 4/ puter ided Manufacturing TCH 4/ Push utton (P) Push utton (P) Scan Timing iagram Scan 3: When rung is scanned the value of PL4 in output data table is still PL in output data table remains When rung 2 is scanned the value of PL3 in ata table is currently value of PL2 in ata table changes to When rung 3 is scanned the Value of P in the input data table is still Value of PL3 in data table remains at When rung 4 is scanned Value of PL2 in the output data table is now so the value of PL4 in the ata table changes to t the end of scan 3 the values in data table are copied to the Physical evices: PL2 and PL4 turn on simultaneously (PL3 remains on) I/O Terminal: I/O ata Table: puter ided Manufacturing TCH 4/ puter ided Manufacturing TCH 4/ Push utton (P) Push utton () ssume rungs are scanned from top - down Scan 4: When rung value of PL4 in output data table is now value of PL in output data table changes to When rung 2 is scanned the value of PL3 in ata table is still value of PL2 in ata table remains at When rung 3 is scanned the Value of P in the input data table is still Value of PL3 in data table remains at When rung 4 is scanned Value of PL2 in the output data table is still so the value of PL4 in the ata table remains at Int Int PL Int Scan Scan 2 Scan 4 Scan 6 Scan 8 Scan Scan 2 Scan 4 t the end of scan 4 the values in data table are copied to the Physical evices: PL turns on (PL2, PL3 and PL4 remain on) Physical Input: Physical : PL PL puter ided Manufacturing TCH 4/ puter ided Manufacturing TCH 4/

9 Push utton () iscrete Input/ Scan : Only the state of changes to ON () during the scan, new state copied at next scan Scan 2: The ON state of is copied into Input data table before Ladder logic is scanned When rung is scanned, is ON, is off so power goes to, ut is off Int is off () When rung 2 is scanned is ON, Power goes thru and Int is On When rung 3 is scanned the Value of in the input data table is now so is energized and contact is ON When rung 4 is scanned, the Value of in the PLC memory is now so the value of PL in the data table changes to Int Int PL n actual PLC has connections to the real" world, and is not just ladder logic. t the end of scan 2 the values in data table are copied to the Physical evices. PL turns on n example hard-wired ladder circuit puter ided Manufacturing TCH 4/ puter ided Manufacturing TCH 4/ Push utton () Programming with NC Contacts Scan 3: No change in the rung output coils When rung is scanned There is continuity thru and but not When rung 2 is scanned no continuity thru top branch ut continuity thru lower branch remains ON Int Int If you want action" (turn ON) when switch is closed (relay energized), use. If you want action" (turn ON) when switch is open (relay de-energized), use. In the rungs, think of the contact as a symbol, = ON = CLOS = TRU = PL = OFF = OPN = FLS = t the end of scan 3 the values in data table are copied to the Physical evices. PL remains on Note: this is probably the most confusing concept in ladder logic puter ided Manufacturing TCH 4/ puter ided Manufacturing TCH 4/ Push utton () Scans 4-5: Int No change in the rung output coils because there is no change in the contacts Int PL t the end of scans 4 and 5 the values in data table are copied to the Physical evices. PL remains on puter ided Manufacturing TCH 4/5335 5