IO Mapping

nXPlc has no process I/O on board: plant inputs and outputs come from the neXo modules on the bus, under the IO-Locali node. The channels of the IO node, described here, do not drive actuators and do not read plant sensors: they tell you how the board is doing, and let you show it to whoever is looking at it.

The rest goes through the CAN bus and the local I/O modules.

Inputs

Channel	Parameter ID	Type	Meaning
PWR_FAIL	1000	BIT	24 V present — active low: FALSE when the supply fails
VCAP_READY	1001	BIT	supercaps charged
TEMPERATURE	5000	REAL	RP2350 die temperature, in °C

PWR_FAIL and VCAP_READY are the pair you shut down on: the first warns that the supply is failing, the second tells you whether you still have the energy to do something about it. They are hardware diagnostics, not process inputs — a comparator watches the 24 V rail and trips before the converter gives way.

PWR_FAIL is active low

The channel reads the nFAIL signal (GP21) without inversion: it is TRUE with the 24 V present and FALSE when the 24 V drops, or when the board is powered from USB only. The alarm is therefore written IF NOT PWR_FAIL THEN: an IF PWR_FAIL THEN that fires the wrong way round is the most common mistake on this board.

Retentive variables: use Persistent Vars

Variables that must survive a power-off go in the application’s Persistent Vars object (Add Object → Persistent Variables): only these are restored at power-up. Variables declared with VAR RETAIN in a program or in an ordinary GVL are not restored.

The firmware saves to flash when the 24 V drops (falling edge on GP21), and only if the values changed since the last save; at power-up it puts them back. The application has nothing to do. On USB power alone there is no 24 V and nothing is saved.

TEMPERATURE is the sensor inside the microcontroller, not a process probe: it measures how hot the silicon runs, not the environment the board lives in.

Outputs

Channel	Parameter ID	Type	Drives
LED_RUN	2000	BIT	green indicator DL6
LED_ERR	2001	BIT	red indicator DL7

The two indicators are bit channels, on or off.