Latching Relay

The Latching Relay node sends 1 to the output when the Set input is set to 1 and sends 0 to the output when the Reset input is set to 1.
When both Set and Reset are equal to 0, the previous state of the output is maintained.

The Behaviour property determines which input must be dominant when both Set and Reset are equal to 1.

The Latching Relay node stores its output state until a new command changes it.

Its basic behaviour is:

• if Set = 1 and Reset = 0, the output becomes 1;
• if Set = 0 and Reset = 1, the output becomes 0;
• if Set = 0 and Reset = 0, the previous output state is maintained;
• if Set = 1 and Reset = 1, the output depends on the configured Behaviour property.

This means that the node acts as a memory element.

The behaviour of the Latching Relay depends on the values of Set and Reset.

When only one of the two inputs is active, the behaviour is always the same:

The last case depends on whether Set or Reset is configured as dominant.

If the Behaviour property is configured so that Set is dominant, when both inputs are equal to 1 the output becomes:

1

The complete logic is therefore:

This configuration gives priority to activation.

If the Behaviour property is configured so that Reset is dominant, when both inputs are equal to 1 the output becomes:

0

The complete logic is therefore:

This configuration gives priority to deactivation.

Assume the following sequence:

The example shows that the node keeps its state even when both inputs return to 0.

Assume that the node is configured with:

Behaviour = Set dominant

and the current output state is 0.

If both Set and Reset become 1 at the same time, the resulting output is:

1

because the Set input has priority.

Assume that the node is configured with:

Behaviour = Reset dominant

and the current output state is 1.

If both Set and Reset become 1 at the same time, the resulting output is:

0

because the Reset input has priority.

The Latching Relay node is useful whenever a command must be memorized until a separate command changes it.

Typical applications include:

• maintaining the ON/OFF state of a device;
• storing an enable or disable condition;
• managing start and stop commands;
• implementing alarm memory behaviour;
• creating bistable logic controls;
• separating activation and reset commands.

The Latching Relay node can be tested using the Logic Module simulation tools.

A typical test procedure is the following:

• start with Set = 0 and Reset = 0;
• verify the initial value of Output;
• set Set = 1 and verify that Output = 1;
• return Set to 0 and verify that the output remains at 1;
• set Reset = 1 and verify that Output = 0;
• return Reset to 0 and verify that the output remains at 0;
• set both Set = 1 and Reset = 1;
• verify that the output follows the configured Behaviour property.

• Use the Latching Relay when the output must preserve its last state.
• The output is not forced to 0 when both inputs are 0; instead, the previous state is maintained.
• Configure Behaviour carefully according to whether activation or deactivation must have priority when both commands are present simultaneously.
• Use Set dominant when activation must prevail.
• Use Reset dominant when deactivation must prevail.
• The node works with digital logical values.

• Logic Module
• Logic Module - Simulation management