Like AC output modules, DC discrete output modules have two main sections: the power section and the logic section.

In a summarized form, the operation of a discrete DC output circuit is:

1. An external power supply is connected to the output card.

2. The card switches the power on or off for each output.

3. The outputs are connected to a device, such as a light bulb.

4. When the output is on, the current can flow to the bulb, completing the circuit.

Discrete output modules are used to turn field output devices on or off.

They can be used to control any two-state device and are available in AC and DC units and in a variety of voltages and current ratings.

DC discrete output modules can have these three output options:

1. Transistor (can be used only to control DC devices)

2. Triac (can be used only to control AC devices)

3. Relay (uses electromechanical as the switching element; can be used with AC or DC devices but have a much slower switching time)

Ranges of DC Output Modules

The ranges of DC output modules represent the voltage DC that the output card sends out. Some common ranges are:

• 12 – 48Vdc
• 24Vdc
• 5Vdc (TTL)

It is important that you determine whether the output module requires a sinking (NPN) or sourcing (PNP) circuit.

Proper connection of DC Output Module

The input circuits for a typical DC discrete output module require either a sinking (NPN) or sourcing (PNP) circuit.

These terms refer to the signal flow relationship between the field input and output devices in a control system and their power supply.

This diagram illustrates the flow relationship between sinking and sourcing outputs to a DC module.

DC input and output circuits are usually connected with a device that has internal solid-state circuitry which requires a DC signal voltage to function.

Field devices that are connected to the positive (+) side of the field power supply are referred to as sourcing devices.

Field devices that are connected to the negative (-) side of the field power supply are referring to as sinking devices.