Andy Kunz asked about measuring isolated voltages.

You could buffer the 3-30VDC to drive a Hewlett Packard 4-20mA loop
optoisolator.

I once had to measure an isolated voltage from a pressure sensor
of a pneumatic valve block.

Initially I tried using the IL300 linear isolator, because I thought
they were clever devices and I wished to learn how to use them for
future ref. When I couldn't get it going in a hurry I fell back to
the plan of opto-isolating the digital signals to/from the serial ADC
(clock, select, data) and this worked fine.

That was a cheap 3-wire serial DAC from TI.
You could use two HP2630 dual optoisolators to isolate an I2C bus
(as I have done), and have multiple isolated devices.

Squeezing I/O data through opto-isolated serial link
uses far fewer isolators than you would if you had to isolate
every digital I/O signal. Its thus a lot cheaper.
I believe Arcom Control Systems do this on one of their
PC104 I/O cards.