Industrial I/O

Digital Outputs

There are a number of digital output formats, but the four major formats are:

• Relay Output,
• Push Pull Output,
• Open Collector (PNP),
• Open Collector (NPN).
• Isolated Transistor Output.
• Triac Output.

Some of these ouputs can have a common terminal, or they can be isolated from each other.

Isolated Relay Outputs
One or more relay contact circuits connected to individual terminals, with no conection to any other circuits. Each relay can control circuits at diferent potentials with the major limitation being the contact rating, terminal rating and pcb layout and spacing.

Non-Isolated Relay Outputs
More than one relay output can be provided with one side of the relay going to a common terminal.

Push Pull Output
The Push Pull output provides a voltage output that is either the positive supply or the zero volt supply. The output is able to sink or source current, but is relative to both the positive supply and the zero supply. This provides both a pull up and a pull down, and can drive a passive input. Typically, it is logic high, that is a high output indicates a '1' and a low output indicates a '0'.

Open Collector PNP
The PNP open Collector circuits use PNP transistors with the Emitters all connected to the common positive supply. The collectors can be connected to load circuits at a different voltage level from the potential applied to the output stage. For example, the output stage can run from +24V and the loads could be 12 volt relays. The positive terminal of the relay supply is connected to the positive terminal of the output stage. This ouput provides a pull up output only, and typically a transistor ON (pull up) indicates a '1' and OFF indicates a '0'. This output is active high.

Open Collector NPN
The NPN open Collector circuits use NPN transistors with the Emitters all connected to the common zero volt supply. The collectors can be connected to load circuits at a different voltage level from the potential applied to the output stage. For example, the output stage can run from +24V and the loads could be 12 volt relays. The zero volt terminal of the relay supply is connected to the zero volt terminal of the output stage.This ouput provides a pull down output only, and typically a transistor ON (pull up) indicates a '1' and OFF indicates a '0'. This output is active low.

Isolated Transistor
One or more isolated transistor circuits connected to individual terminals, with no conection to any other circuits. Each transistor can control circuits at diferent potentials with the major limitation being the transistor rating, terminal rating and pcb layout and spacing. The transistors are typically designed for controlling max 24VDC circuits at less than 100mA. The transistor output can control DC only and is polarity sensitive.

Triac
One or more triac circuits that can be individually isolated, or may have a common terminal. The triac can only be used to contol AC circuits. The voltage is typically limited to 250VAC and the current is typically less than 1A. There is protection circuitry in parallel with the Triac which will result in a residual current flow. It is therefore important to ensure that the switched load has a sufficiently low impedance that the residual current has no impact. Neon lamps will iluminate on the leakage current.