General Purpose Inputs

The I/O controller manages:

Configuration

The inputs are initially configured at power-on as follows:

The initial configuration is suitable for both TTL signaling and contact closure devices. Alternate threshold voltages can be selected, for each input style individually through the Virtual File System API.

Threshold

Threshold voltage

Notes

TTL

1.5 V

For TTL signaling and contact closure devices. This is the default configuration.

CMOS

2.5 V

For 5V CMOS signaling.

12V

6.0 V

For 12V signaling.

Alternate time constants can be selected, for each input filter individually, through the Virtual File System API.

InputFilter

Time constant

Notes

Off

Filter is turned off.

10ms

10 ms

Filter is turned on with a 10ms time constant .

100ms

100 ms

A 100ms filter is very efficient to reject 50/60Hz perturbation. This is the default configuration.

Alternate impedance thresholds cannot be selected.

State

The I/O controller measures both the voltage and the impedance across every input at a rate of 200 Hz. It compares the measured values with the threshold and determines the input state according to the following resolution table:

Measured impedance

Measured voltage

State

Below impedance threshold

Below voltage threshold

Low

Below impedance threshold

Above voltage threshold

High

Above impedance threshold

Don't care

Disconnected

The input state is reported through the Virtual File System API. The value must be interpreted differently according to the type of device attached to the input:

Level

Meaning for contact devices

Meaning for logic devices

Low

Contact is closed.

The logic level is 0.

High

N/A.

The logic level is 1.

Disconnected

Contact is open.

The line is disconnected.