Program Control Instructions

Transcription

1 Program Control Instructions Industrial Controls University of Akron Overview Master Control Reset and Master Control Relay Jump and Subroutine Instructions Immediate Input and Output Forcing Input and Output Safety Circuits Interrupts

2 Overview Fault Routines Debug Tool Master Control Relay Electro-mechanical relay, not PLC Used to provide safety to equipment including PLC

3 MCR 1 Instruction MCR rung = True : Executes rungs in MCR zone based on each rung s individual input condition (as if the zone did not exist). MCR rung = False : Resets all nonretentive output instructions in MCR zone regardless of each rung s individual input conditions. 1 Master Control Reset Example of MCR

4 Example with Retention Multiple MCR Instructions MCRs can not be nested Multiple MCRs must be non overlapping Multiple MCRs permitted Every MCR start rung (fence) must be followed by an unconditional MCR stop rung (fence)

5 Pitfalls Do not overlap MCR zones. Note timers and counters within MCR are disable when MCR zone is disabled. Do not jump into MCR zone because this will activate disabled zone. Must end MCR with unconditional MCR rung (no input instructions). Start MCR must be last instruction on rung. JMP and LBL Instructions Jump instruction JMP is output instruction (coil) Warning do not jump into MCR zone Label instruction Input instruction on rung Must be first instruction on rung Multiple jumps to same label permitted

6 Reference Direction -]LBL[-- -]LBL[-- Forward Reference -(JMP)- -(JMP)- Backward Reference Pitfalls Watch backward reference because endless loop can cause watchdog timer fault. JMP and LBL are the goto statements of RLL and should be used sparingly to prevent spaghetti ladders. Intervening rungs not scanned Timers and Counter don t update

7 JMP Example JSR, SBR, and RET Instructions JSR is output instruction which initiates call to subroutine ladder file. SBR optional is entry point of subroutine call in ladder file and must be first instruction on first rung. RET is output instruction which returns control to rung following JSR rung.

8 Example -(JSR)- LAD2 Main Program -]SBR[-- -(RET)- LADm Subroutine Optional --(END)-- also causes return Passing & Returning Parameter

9 Pitfalls Subroutines may be nested but number of levels depends on processor May have multiple JSR s per SBR but remember common database shared for each subroutine invocation. Nested Subroutines

10 Immediate I/O Instructions IIM or immediate input with mask output instruction allows slot to be immediately read. Mask bit = 1 allows immediate update of input database and 0 blocks update. IOM or immediate output with mask output instruction allows slot to immediately be written. Mask bit = 1 allows immediate update of input database and 0 blocks update. REF or refresh output instruction interrupts ladder scan and updates all Input and Output Forcing Forced Inputs done after input scan and effect all of the program ladder logic Forced Output only effects addressed output terminal (not OTE instructions associated with forced output). Safety first and safety last when forcing input and output.

11 MCR Master Control Relay Seal-In Using Auxiliary Contact

12 N.O. Stop Button N.C. Stop Button is Bad Design Selectable Timed Interrupt Status word 30 is time base in millisecond intervals Status word 31 is ladder file number (zero to disable) Ladder file normally uses IIN and IOT STD and STE used to bracket critical sections of ladder logic

13 Critical Code Example Fault Routine Allows controlled shut down Status word 11 Recoverable fault routine scanned and return to main program Nonrecoverable fault routine scanned and PLC shut down None present PLC shut down

14 Fault Routine Debug Instructions TND or temporary end output instruction replaces function of END instruction. Rest of ladder not scanned. SUS or suspend output instruction throws processor in to suspend idle state with all outputs disabled

15 Examples Master Control Relay Traffic Light with jumps and MCR 2/24/