Chapter 13

Basic Relay Instructions

Objectives (1 of 2)

• Describe the function of the normally open, or examine if closed, instruction.

• Describe the function of the normally closed, or examine if open, instruction.

• Explain the function of one-shot instruction.

Objectives (2 of 2)

• Explain the function and programming of the latch and unlatch instructions.

• Explain input and output instruction formatting for the SLC 500 and MicroLogix PLCs.

• Given an address, identify the input or output point on an SLC 500 fixed or modular PLC and a MicroLogix 1000 PLC.

PLC Instructions

• Each PLC manufacturer has its own vocabulary of instructions called the PLC instruction set.

• Even though different PLCs have different instruction sets, there are basic instructions shared by all PLCs.

Bit or Relay Instructions

• Contacts and coils are the basic symbols found on a ladder program.

• Normally open and normally closed instructions are programmed to represent input conditions.

• Contacts and coils are referred to as relay instructions.

Overview of Bit Instructions

SLC 500 XIC Instruction

• SLC 500 normally open instruction is called the XIC or examine if closed instruction.

• XIC instruction directs the processor to test for an on condition from the referenced address bit.

XIC Instruction Interaction

XIC Input Instruction

SLC 500 XIO Instruction (1 of 3)

• SLC 500 normally closed instruction is called the XIO or examine if open instruction.

• XIO instruction directs the processor to test for an off condition from the referenced address bit.

SLC 500 XIO Instruction (2 of 3)

• The XIO instruction is normally closed, representing a 0 in the input status table.

• Finding a 0 in the status file referencing the instruction address means the device controlling the bit address is in the off condition.

SLC 500 XIO Instruction (3 of 3)

• A normally closed instruction evaluated as closed is a true instruction.

• Finding the normally closed instruction true, the instruction will continue to provide continuity through the instruction on the rung.

Instruction Controlling an Output Instruction

Physical Input Conditions and the Normally Closed XIO Instruction

SLC 500 Output Instruction (1 of 2)

• It is typically represented as an output coil.

• SLC 500 refers to an output enable or OTE instruction.

• Every rung must have an output instruction.

• Multiple outputs must be programmed in parallel.

SLC 500 Output Instruction (2 of 2)

• Output instruction is always the last instruction before the right power rail.

• Output instruction represents the action to be taken when the solved input logic results in a logically true or false rung.

Ladder Rung Containing XIC and OTE Instructions

Bit Instruction Addressing

• Each instruction on a ladder rung must have an address that associates with the field device and data table the instruction is examining.– Typical XIC address is I:1/0.– Typical OTE address is O:2/1.

Basic SLC 500 Input or Output Addressing Format

I:1/0 Breakdown (1 of 2)

• I = This is an input instruction. – Input address data stored in the input status

file

• : = element delimiter – Separate file type and file number

• 1 = Identifies the chassis slot in which the address’ module resides

I:1/0 Breakdown (2 of 2)

• / = bit delimiter– Separates the input bit slot reference from the

input bit reference

• 1 = screw terminal number of this input reference from the input module residing in chassis slot 1

Slot Identification in an SLC 500 Chassis (1 of 2)

• To accurately identify a specific input point among multiple input modules, each slot in a modular chassis is assigned a slot number.

• Power supply mounts on the left side of the chassis.

• First slot next to power supply is reserved for the processor.

Slot Identification in an SLC 500 Chassis (2 of 2)

• Processor always goes in the first slot, which is identified as slot 0.

• Slots are numbered in decimal numbers from left to right: slot 0, 1, 2, 3, 4, 5, 6.

• Four chassis are available.– 4-, 7-, 10-, and 13-slot chassis– Total 3 chassis 30 local I/O slots per PLC

Seven-Slot SLC 500 Chassis Slot Identification

The One-Shot Rising Instruction

• One-shot rising instruction (OSR) is an input instruction that allows an event to occur only once.

• OSR instruction fires on the rising edge (off-to-on transition) of the input pulse.

• It will not fire again until input transitions to off and then on.

SLC 500 Family One-Shot Rising Ladder Rung

Leading-Edge Versus Trailing-Edge One-Shot Timing Diagram

Internal Bit Controlling a One-Shot Instruction

Output Latch and Unlatch Instruction (1 of 2)

• An output latch (OTL) instruction is an output instruction used to maintain, or latch, an output on even if the status of the input logic changes.

• The output unlatch (OUT) instruction is used to unlatch the latched output.

Output Latch and Unlatch Instruction (2 of 2)

• OTL and OTU instructions are typically used in pairs.

• OTU is used by itself to unlatch status bits set by the processor.

• OTL and OUT are retentive instructions.

Retentive Instructions (1 of 2)

• Two types of instructions– Nonretentive– Retentive

• Non-retentive instructions do not retain their logical state through a rung true-to-false transition or power interruption.

• Retentive instruction retains its logical state through a rung transition or a power flow interruption.

Retentive Instructions (2 of 2)

• Retentive must have battery backup to retain state through power interruption.

• OTE instruction is non-retentive.

• OTL and OUT instructions are retentive.

Latching and Unlatching Ladder Logic

Latching Instruction Programmed Before the Unlatch Instruction

Internal Bit B3:0/0 Used as an Output

Each Bit File Element Consists of One 16-bit Word

Bit File Addressing (1 of 2)

• B = a bit instruction– Bit file address data is stored in the binary or

bit file, B3.

• 3 = identifies this as file 3

• : = element delimiter – Separate file type and file number

• 1 = identifies the element number

Bit File Addressing (2 of 2)

• / = bit delimiter– Separates the file designator from the bit

number

• 1 = bit number of this reference from bit file element 1

User-Defined Bit Files

• If your processor supports expanding data files and has enough memory, any data file over file 8 up to 255 can be created as a user-defined bit file.

• Each user-defined file can have up to 256 elements.– Sample addresses: B10:4/8 or B65:29/12