The discrete module channels of our product use an integrated output stage that implements FET circuit diagrams, as described in the operations manual(s). These channels can be programmed for output with multiple drive formats, or for input as either direct voltage sense or contact sense with a programmable pull-up/down current source. Each channel has two output drive FETs – high side and low side – that allow it to be set up as a current source, current sink, or push-pull output. A voltage sense circuit is present on the output side of the drive FETs for each channel, which is utilized as the input read circuit. This allows individual channels to be used as contact or direct voltage sense inputs.

Additionally, each channel has a "wrap" circuit that handles a specific channel complement while being scanned or multiplexed. The external VCC and GND applied are used for output drive switching and biasing for the drive FETs. When programmed for output, the "wrap" circuit compares the commanded output with the actual voltage read on the I/O pin against thresholds programmed to determine the state. When programmed for input, the "wrap" circuit acts as a redundant read, and both level reads must agree.

If VCC and GND are removed, this only shuts down the output drive and pull-up/down current source. The control and "wrap circuits" remain active and expect the channel(s) to be online and operational.

Built-in-test (BIT), utilizing the wrap, is typically used in an operating system, where the system is initialized, VCC and GND are applied, the loops are on, and the I/O is being operated.