“Discrete” is a term that describes inputs and outputs that have two possible states… On and Off. This is in contrast to “analog” I/O which have a range of levels. An example of an analog input is a 4-20mA analog current from a flow transmitter or a temperature from a thermocouple probe. A discrete input might be the On/Off state of a limit switch or an On/Off output from a PLC. The terminology for the various types of “Discrete” inputs and outputs can be confusing. The following information can be used in conjunction with the SureCross Node data sheets to understand the wiring configurations for the Node discrete Inputs and Outputs.

Discrete Input Types

Many of the Nodes are available with discrete inputs. Variants of discrete inputs are offered in different Node models.

Programmable for Sourcing/Sinking Input Signals
These Node inputs can be user configured in the field to accept two different input types from an external device which are referred to as sourcing or sinking input signals. For example, the DX80N9X6S8P4 Node has 8 discrete inputs that are field programmable to interface with Input Sourcing or Input Sinking signals from external devices.

Sourcing Input Signal
A Node input configured as a Sourcing input requires the external device to source a driving voltage /current into the Node input to effect a state of On or Off. An example of a Sourcing input is a PLC or controller output that sources a 24Vdc output when ON and a 0Vdc output when OFF. A Node input configured for a Sourcing input signal could be used to sense the state of the PLC. Note that Sourcing Input signal devices must share the Node Ground terminal. Node inputs configured to accept Sourcing inputs typically have the maximum drive current specified (ie: the maximum current that the Node input might draw from the signal source) as well as the voltage thresholds that the Node input circuitry defines as High or Low inputs.

Sinking Input Signal
A Node input configured as a “Sinking Input” merely requires the external device to short the Node input to Node ground (ie “sinks current”) to assert one state (eg ON) and allows it to float to assert the opposite state (eg OFF). This mode is commonly used with switch inputs. When the switch is closed, the Node input is shorted to ground. When the switch is open, the input is allowed to float. Node inputs configured to accept Sinking inputs typically specify a maximum voltage that the Node input will consider to be a Low signal.  

Discrete Output Types

Many of the SureCross Nodes are provided with discrete outputs as well. Nodes are provided with either Sinking or Sourcing discrete outputs for connection to external devices or loads.

Sourcing Outputs
Sourcing outputs provide (ie source) an output voltage. As transistors are typically used to control the discrete outputs and they are not perfect switching devices, the ON voltage is a function of the current flowing to the load and the OFF voltage will not be 0 volts due to slight current leakage. Specifications of these Sourcing outputs include a minimum ON or “high” voltage spec as well as what the maximum OFF or “low” voltage output will be during operation. Additionally, the maximum current that an output can source is also specified. For example, the DX80N2X6S6P6 Node has six sourcing outputs that can deliver up to 100mA of current at a voltage which is within 2 Vdc of the supply. The OFF state voltage is specified to be less than 2Vdc. An LED or light bulb are examples of loads that might be driven by sourcing outputs.

Sinking Outputs
Sinking outputs short (ie sink) current from an external load to ground when ON. Typically an NPN or NMOS transistor is used for sinking outputs and the configuration is commonly referred to as an “open collector” or “open drain” configuration. Sinking outputs turn on the output transistor routing load current to ground when ON and the transistor is turned OFF when off disallowing current to flow from the connected load to ground. As transistors are not ideal devices, when they are ON the output is not shorted completely to ground and when OFF, there will still be a bit of leakage current flowing. Specifications for sinking outputs typically include the saturation (or maximum) voltage that will be seen at the output when the output is ON and a specified current if flowing. The more current, the higher the saturation or ON voltage will be which is an inherent characteristic of transistors. Note that when OFF, a slight current will still flow which is also specified for the Node. Commonly, heavier loads are controlled using sinking outputs since the Node power supply does not have to supply the load current. Typically, the current flowing through the load and into a sinking output will be provided external to the load.