PLC INSTRUCTIONS
Functions and InstructionsRelay-type (Basic) instructions: I, O, OSR, SET, RES, T, C
Data Handling Instructions:
Data move Instructions: MOV, COP, FLL, TOD, FRD, DEG, RAD (degrees to radian).
Comparison instructions: EQU (equal), NEQ (not equal), GEQ (greater than or equal), GRT (greater than).
Mathematical instructions.
Continuous Control Instructions ( PID instructions ).
Program flow control instructions: MCR (master control reset), JMP, LBL, JSR, SBR, RET, SUS, REF
Specific instructions:
BSL, BSR (bit shift left/right), SQO (sequencer output), SQC (sequencer compare), SQL (sequencer load).
High speed counter instructions: HSC, HSL, RES, HSE
Communication instructions: MSQ, SVC
ASCII instructions: ABL, ACB, ACI, ACL, CAN
Internal Relays
Auxiliary relays, markers, flags, coils, bit storage.
Used to hold data, and behave like relays, being able to be switched on or off and switch other devices on or off. They do not exist as real-world switching devices but are merely bits in the storage memory.
Internal Relays Use
In programs with multiple input conditions or arrangements. For latching a circuit and for resetting a latch circuit. Giving special built-in functions with PLCs.
Retentive relays (battery-backed relays)
Such relays retain their state of activation, even when the power supply is off. They can be used in circuits to ensure a safe shutdown of plant in the event of a power failure and so enable it to restart in an appropriate manner.
Latch Instructions (Set and Reset)
The set instruction causes the relay to self-hold,, i.e. latch. It then remains in that condition until the reset instruction is received.
The latch instruction is often called a SET or OTL (output latch).
The unlatch instruction is often called a RES (reset), OTU (output unlatch) or RST (reset).