Mesa 7i96 mixed voltage field input wiring

PCW wrote:

Yes, the 7I96/7I96S isolated input circuits are identical.

To use mixed I/O voltages you need sinking inputs
(7I96S isolated input common connected to
negative of field supplies and inputs connect to +
to activate)

The main disadvantage I see with using 5V field wiring
is an inadvertent short to say +24V...
 

Why would you need to use sinking inputs? According to the manual +4v to +36v is the range for sourcing inputs. You would generally  connect all power supply commons then chassis ground them. Their shouldn't be a problem with sourcing inputs of mixed voltages >+4v and <+36v right as long as everything is sourcing and supplies are commoned together and that DC common is the 7i96 input common?? The only time sinking devices are absolutely necessary would just be the case of using a transistorized(word?) switch some input that was not voltage tolerant to the supply voltage of the device that could otherwise have a PNP output.

You need to use sinking inputs with mixed field voltages for this reason:

The isolated inputs respond to +-4 to +-36V in difference between the input common
and the input.

For sourcing inputs (say input common connected to +24V) the inputs
would work with 24V signals, but not 5V signals (the inputs would only turn
off at +20V and above)

Edit: Yeah you're right, please excuse my brainfart

I've been using a 7i96 for a couple years so I haven't looked at the manual in a long time but since there's some confusion now at least for me, the manual is throwing me off a bit here too.....

ISOLATED INPUT CHARACTERISTICS
The isolated inputs use opto-isolators with a 4.7K input series resistance. This
results in an approximate current draw of 5 mA at 24V. The inputs will operate with +-5V
to +-36V signals relative to input common. Isolated inputs are relatively slow and not suited
for signals faster than about 5 KHz.
For PNP type sensors or switches with a common positive, the input common pin
is grounded and the sensor or switch applies a positive voltage to the input pin to activate
the input.
For NPN type sensors or switches with a common ground, the input common is
connected to +5 to +36V and the input pins are grounded to activate an input. 

PNP type switches would not have an implied common positive in any machine, they could but that's not a give in. If all it's doing is activating and optocoupler which I's assume is an IC with multiple independant optos and maybe steering diodes or something to get the right polarity to the LED, realistically I suppose the inputs are looking for a 4v difference from the reference to activate the LED as long as all inputs move in the same direction from the input common reference.

I realize that's a bit tough to word in a manual people need to understand but I'm still uncertain of what the problem would be as long as the inputs can activate and can tolerate each individual input voltage.

Assuming a 24V field voltage and conventional current flow for simplicity.

Sourcing inputs have the input common tied to +24V (so they "source" current)
They are activated by connection to ground (a sinking or NPN type sensor)

Sinking inputs have the input common tied to ground (negative of the 24V supply so they "sink current)
They are activated by connection to +24V (a sourcing or PNP type sensor)

A sinking sensor must be matched with a sourcing  input and vice-versa.

Maybe a diagram will help:


(Diagrams such as these could make a huge improvement to the Mesa manuals IMO.)