Tim Ganz
Member
How do I choose between Isolated and non Isolated analog modules? Where would I use one or the other and why. Need to add some analog outs to a small 1756 system and not sure what to look at.
1756 Analog Output Modules
- Thread starter Tim Ganz
- Start date
rdrast
Lifetime Supporting Member
If you need isolated inputs and outputs, you need an isolated module. If you don't need isolated I/O, you don't need an isolated module.
Put another way: If you can tie the commons of different things together without seeing a big flash and a puff of smoke, you probably don't need isolation. If there is a big flash and smoke, you do.
Isolation can also be used to break up ground loops, if there is that problem.
Honestly, if I need isolation, much prefer to use an isolator external to the PLC. That way, in the future, someone stumbling along in the cabinet has less chance of just moving the wires to a different module/output as a troubleshooting step and releasing the flash/smoke.
Put another way: If you can tie the commons of different things together without seeing a big flash and a puff of smoke, you probably don't need isolation. If there is a big flash and smoke, you do.
Isolation can also be used to break up ground loops, if there is that problem.
Honestly, if I need isolation, much prefer to use an isolator external to the PLC. That way, in the future, someone stumbling along in the cabinet has less chance of just moving the wires to a different module/output as a troubleshooting step and releasing the flash/smoke.
curlyandshemp
Lifetime Supporting Member
Also, if there is mix of active and passive devices, best to put on isolated ins/outs
Hopefully my previous experience will help. I think you need to know what the device receiving the analog output signal is referencing that signal to. If all of the devices reference the signals to same ground as your analog module, then non-isolated is fine.
I recently had a non-isolated analog input module receiving signals from various devices. One of the devices was powered from a separate 120VAC source and it generated a signal which was referenced to some odd voltage (like 7VDC?) rather than ground. I needed a signal isolator for that input to keep things happy. Without the isolation, my ground reference probably would've been bouncing all over the place causing havoc.
I recently had a non-isolated analog input module receiving signals from various devices. One of the devices was powered from a separate 120VAC source and it generated a signal which was referenced to some odd voltage (like 7VDC?) rather than ground. I needed a signal isolator for that input to keep things happy. Without the isolation, my ground reference probably would've been bouncing all over the place causing havoc.
curlyandshemp
Lifetime Supporting Member
An active analog device is one that has its own internal power supply, there fore the signal is powered.
A passive device requires external power . Connecting multiple passive devices to the same analog input rarely causes issues as the power source is common for all if the power is supplied from the PLC panel. Most pressure and level sensors are passive.
Active devices can be flowmeters, PH analyzers usually all powered by AC.
widelto
Member
Maybe I'm hijacking this thread but this happens to me recently:
I ordered a HART Flex I/O board 1794-IE8H, channels on this board are not isolated and I have 8 isolated 4-wires HART flowmeters. 1794-IE8H only works with non isolated HART devices. I have to order a 1794-IF8IH, channels on this are isolated and now all my flowmeters HART data is present and working properly.
I ordered a HART Flex I/O board 1794-IE8H, channels on this board are not isolated and I have 8 isolated 4-wires HART flowmeters. 1794-IE8H only works with non isolated HART devices. I have to order a 1794-IF8IH, channels on this are isolated and now all my flowmeters HART data is present and working properly.
Boils down to cost - I would always use isolated modules...
V0N_hydro
Member
I always use isolated modules. Saves worrying about which devices are on which modules, headaches when the devices that show up on the machine are different than they were supposed to be, and allows some flexibility when you need to do something weird like run the same analog through two different supplier's PLCs.
the m340 8 channel isolated analog input module is ~$800 CAD I think? cost savings on the non isolated module can't be more than a couple hundred bucks, only takes one whoops to eat that up
the m340 8 channel isolated analog input module is ~$800 CAD I think? cost savings on the non isolated module can't be more than a couple hundred bucks, only takes one whoops to eat that up
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