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- d - digital type points
as we now have PID control points and analog indicator/alarm points we need
something more: Digital Control Points.
that DCP is just a word, what means:
1 .. N digital inputs can be used to form bit patterns what can be assigned to show a PV
( 01 STOP, 11 RUN, 00 FAULT )
using only one input you can assign 2 words, a ON word and a OFF word.
and optionally can configure one if them to be the ALARM word,
( like: 1 "ok", 0 "( temperature oven ): HIHI" alarm )
( here see again, good thinking, wire break gives alarm )
( so DCP also covers a digital in indicator and a digital in alarm type )
add you can assign 1 .. M digital outputs ( latching ) and bit patterns with SP words.
SP1 0 (STOP)
in case one digital output and no digital input its a usual Dout, ( with 2 assigned SP words )
but with inputs ( feedback ) following feature is possible.
operator action: select a SETPOINT
DCP drives the corresponding bit combination to the assigned outputs.
after a configured timeout the input combination (PV) is checked and if not match
a alarm is generated.
SP START, after 6 sec. PV != RUN >> FAIL ALARM.
also if after successful "start" later the PV changes like PV RUN to PV FAULT
also a FAIL ALARM is created.
And at last even a SP sequence can be generated ( each with timer )
operator select SP START
-1- DCP drive SP 01 RESET
-2- if PV after timer is not 01 STOP ( like 00 FAULT ) alarm
if ok in time: drive SP 10 START
-3- if PV after timer is not 11 RUN alarm
on SP START drive 10, after timer drive 00
to create pulses from latching outputs.

i know you all know, there are digital input and digital outputs
and they can be low or high. But i hope above thinking
of DCP is useful to deal with it in a more structured way.

you think that i talk old fashioned electric? a Motor Control Center with 3 inputs and 2 outputs "M32" wired to a I/O card?
yes and no, because if today the motor, inverter.. is connected by fieldbus...
you have a SP (output) word and a PV (input) word by link, and the operation
software is still same, mapped to selected bits of these words.
and a pneumatic diverter valve with 2 proximity sensors for the position ( Valve 2 inputs, 2 outputs "V22" ) still might need digital wiring old style.

but for the digital I/O same as with the analog I/O, a output of 3.3Volt, can drive 15mA,
and a input 3.3V, damaged by 5V already?,
(with schmitt trigger 0.15V) see low at below 0.3*3.3V and high above 0.7*3.3V??
need to check that with DAC later,
its all far from industrial / electrical standards and need to be beefed up for power electric.