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flyers
December 17th, 2006, 09:15 PM
Dear all,

I'm looking at the Flex IO analog input, 1794-IE8 & 1794-IF4I, what is the different between the isolated & non-isolated?

The 1794-IE8 should be using a 1794-TB3 or TB3S? I'm mixed up with the manuals as per attached. Referring to the table.

Thank you.
http://www.plctalk.net/qanda/uploads/12.JPG

Singapore_Mats
December 18th, 2006, 01:26 AM
for me, isolated should be that signal ground is not connected to power ground.

Tom Jenkins
December 18th, 2006, 09:38 AM
I'm not familiar with the Flex I/O stuff, but in general isolated in this context refers to analog I/O that does not have the power supply "-" tied to gether on all transmitters or all points on the I/O card. If you are using multiple loop powered transmitters on a single power supply, they are not isolated.

In many cases you have to look carefully at the I/O card as well. Many analog inputs are inherrently non-isolated, since they have all of the "-" terminals on the card internally connected.

flyers
December 18th, 2006, 07:27 PM
Thanks for the replies.

If I understand correctly, isolated means the '-' signal is individual among all the channels, while non-isolated then '-' signal are connected with each other.

So the advantage of isolated is, when there is a short circuit at a particular channel, only that particular channel will be affected, the rest will still be okie? Am i right?

Black Jack
December 18th, 2006, 07:41 PM
Typically on analog input modules, isolated means separate analog to digital converters for every channel and every channel can be sourced from a different power source. Non-isolated inputs share a common converter and are referenced to the same common voltage.

Singapore_Mats
December 18th, 2006, 07:43 PM
yes, and sometimes you want to measure temperatures in different buildings and dont want any current to leak, or in my case where a ship can have DC24V, AC115V and AC230V, all used at the same time.

flyers
December 18th, 2006, 07:57 PM
yes, and sometimes you want to measure temperatures in different buildings and dont want any current to leak, or in my case where a ship can have DC24V, AC115V and AC230V, all used at the same time.

Hi,

This means different operating voltage of the analog devices can be connected to the same analog input module?

But, I taught analog module can accept 4-20mA (of course can accept voltage like 0 -5 VDC...etc..), no matter what is the operating voltage for the devices as long as the analog device is giving 4-20mA I can connect them into the analog module? NO?

Singapore_Mats
December 18th, 2006, 08:17 PM
yes, you can connect all of them into one analog module, but if you have one analog coming from an office and one from workshop next to a welding-machine or big motor... then its a good idea to have them isolated.

rPraveenkum
December 18th, 2006, 10:24 PM
If the common of the all analog inputs shorted is Non-Isolated & the no connection between the one analog input channel to the other then it is called isolated Input

Ken Moore
December 19th, 2006, 04:59 AM
Isolated inputs are needed if you have any 4-wire instruments. Each 4-wire will usually supply it's own power, so they don't share a common.

Tom Jenkins
December 19th, 2006, 09:56 AM
"Isolated inputs are needed if you have any 4-wire instruments. Each 4-wire will usually supply it's own power, so they don't share a common."


Actually, Ken, that isn't necessary. We have often mixed 2-wire (such as a pressure transmitter) and self powered transmitters (such as a 120 VAC powered analytic instrument) on the same analog card without problems.

If there is a return wire for the signal it all works out OK. That's because a loop powered device is like a current sinking device, constantly changing it's internal impedence so the current through it is proportional to the measured parameter. It isn't necessary for the output current to the PLC card to be returned to the transmitter to have it function.

A self powered device is current like a current sourcing device, where it actually modulates the output current directly to make it proportional to the measured parameter. It isn't necessary to have the signal current through the PLC card actually return to the device for proper function.

flyers
December 19th, 2006, 10:30 PM
Thank you for all the replies, appreciate that.