mrowen,
As Ron already indicated, current flowing in one direction is DC.
That is the definition of DC.
Whether the current is pulsating between zero-current and some peak-current, as in simple half-wave rectification with no filter capacitors, or pulsating between one DC value (of a given polarity) and another DC level (of the same polarity), that current is, by definition, Direct Current.
Granted, it can be shown that the algebraic summation of a particular set of True AC Signals can produce a DC waveform.
However, it doesn't matter what complex waveforms might have been used to generate the result... if the resulting waveform is DC, then the final waveform is DC.
The "alternating", or rather, "fluctuating" portion of a DC signal can indeed be stripped off, through a filter, thus producing an AC-like signal. Of course, whether or not that ends up being a true AC signal depends on the bias signal onto which the stripped signal is applied.
Now, as far as 60Hz being applied to a simple diode, the result will produce a waveform that has equal ON-time and OFF-time; .0166 Seconds ON followed by .0166 Seconds OFF, and so on.
If that pulsating signal is applied to an LED, some of us can see the flicker, some of us can't. I have a lower retention factor. Having a lower visual retention factor means that one is more likely to see things as they really are.
At the same time, the LED has a particular "delivery retention". This is more apparent in full-wave rectification. That is, light emission from the LED does not go OFF completely before it is told to turn back ON. In this case, the light information from the LED is more like a rippled DC than a true full-wave rectified signal.
If the signal is from a 50Hz source (.020 Seconds ON follwed by .020 Seconds OFF), as in the European system, the flicker is more apparent.
Whether or not one can discern the flicker depends on ones' eyeball.
One of the aspects of visual acuity is "retention". Most people have high visual retention. That is, if a light goes ON-OFF-ON, at some particular frequency, their eyeballs do not notice the ON-OFF-ON transitions. Their retention factor is high. For those that have a high(er) retention factor, the eyeball/brain connection tends to smear the flow of light information from one video frame to the next.
For some of us, television in America (and other 60Hz nations) is easier to watch than the television in 50Hz nations (typical of Europe). Sometimes, European television gives me a head-ache.
BUT,... all of that that is neither here, nor there... at least, not in terms of a photo-detector. A photo-detector will indeed "notice" the flicker. Whether or not the "flicker" has an impact on the "reading" depends on the extent of the flicker.
But now, in terms of the original question...
Since the field devices are PNP...
I don't belive there will be any problem reading either one or two inputs being ON since the internal resistor in the PLC input should have been sized to accomodate zero-resistance on the field device (as in the case of a simple limit switch with zero-resistance).
In terms of what the PLC "sees", you don' need no stinkin' diodes.
Blah, Blah, Blah... so what else is new?