Relay Instructions for Allen Bradley’s PLC Ladder Diagram
PLC 

The use of relay instructions is to regulate and control the status of bits in a data table, such as input bits or bit words for timer controls.

These instructions are:

  • XIC controls the bit for the ON condition
  • XIO controls the bits for the OFF condition
  • OTE holds bit ON or OFF (non retentive)
  • OTL locks bits on ON (retentive) conditions
  • OUT returns (unlatch) bits in OFF (retentive) conditions.

Program instructions on the ladder bit logic:

Logic Rung

The instructions are executed in each situation, the bit address is tested according to certain conditions whether the condition is ON or OFF.

Rung consists of continuous lines with correct instructions starting from the beginning of the stage until the output is activated (enabled).

Normally open (Mnemonic: XIC):

When the component circuit is closed, the module whose input terminal is connected to the equipment will detect the closed circuit. Reflexively the processor will give an ON statement to the table data. When the processor gets the XIC instruction with the address in accordance with the input terminal, the processor will determine which component will be closed. If the processor gets an ON statement, then the logic instruction will be set to “true”. If the processor gets an OFF statement, the logic instruction will be set to “not true (false)”.

Example:
If the bit I: 1/0 conditions are specified ON, then in the input table it will be set to “true”. This bit corresponds to terminal number 0 in the input module in slot 1. If the input circuit is “true”, the instruction will also be “true”.

Normally closed ( Mnemonic: XIO)

When the component circuit is opened, the module whose input terminal is connected to the equipment will detect the closed circuit. Reflexively the processor will give an OFF statement to the table data. When the processor gets the XIO instruction with the address in accordance with the input terminal, the processor will determine which component will be turned off (open). If the processor gets an OFF statement, then the logic instruction will be set to “true”. If the processor gets an ON statement, then the logic instruction will be set to “not true (false)”.

If the condition of the XIO instruction in the rung is “true” (the input is open), then the processor can activate the output instruction. If the XIO instruction is “false” (closed input), the processor cannot activate the output instructions.

To test true or false instructions depends on the state (on or off) of the bit address specified by the processor.

Example:

If bit state I: 1/0 is set OFF, then the input table is set to “true”. This bit corresponds to the terminal number 0 in the input module contained in slot 1. If the input circuit is “false”, the instruction will be “true”.

Output  (Mnemonic: OTE)

OTE instructions are used to control bits in memory. If the bit corresponds to the terminal module output, then the equipment connected to this terminal will be energized when the instruction is active and de-energized when passive instructions.

If the circuit state is “false”, then the components associated with the circuit will be de-energized. The OTE instruction is the same as the relay component, where this instruction is controlled by the previous input instructions. OTE instructions are forwarded to the processor to control the bits in the space.

OTL and OTU:

OTL instruction is a restrained output instruction that can only turn bits off, but cannot turn it off. These instructions are usually used in pairs with OTU instructions.

If the processor mode is switched from the RUN position to the PROG, or when the processor loses power (and there is a back-up battery for memory), and if the last condition of this instruction is true, then this condition will still be maintained even if the conditions are true which controls the output latch instruction changes to false.

Unlatch (OUT) output instructions are informed to the processor to turn off the address bits in space. After that the bit remains OFF no matter what the conditions are, until it is specifically turned on by OTL instructions on another space.

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