Mesa 7i96 mixed voltage field input wiring

Hi,

A quick query, probably a silly question but wanted to get confirmation before continuing with this.

I have a situation where in need to mix both 5V and 24V voltage levels on the isolated inputs on the top section of the Mesa 7i96. I have proximity sensors at 24V and some other inputs from optical switches at 5V levels.

I could move the 5V stuff to general GPIO pins but I'd prefer to leave these as is if possible as it would require significant rewiring in my situation.

Am I okay to common connect the GND's from both the field 5V and 24V PSU's to the input common GND? (proximity switches are NPN so it will work from that perspective in my case).

Just to note that the main Mesa card is on a separate 5V supply from the field wiring noted above just to clarify (it has it's own 5V supply not used elsewhere) so it shouldn't compromise the isolation, so to speak. 

Thanks in advance, John

That should be fine

Great, thanks PCW.

I have just recently ordered a 7i96s and I'm trying to get my ducks somewhat lined up prior to its arrival. I am just wondering if the 7i96s would tolerate the same ie. commoned supplies like the 7i96.

Also, would it be OK to use a single 5V supply for both the Mesa supply and some external opto isolated field IO along with the 24V for the other field wiring?? Or is that just asking for trouble?

thanks in advance.





 

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...

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.