Amr Hassan said...
"please advise how can i connect a two wire transmitter to redundant anlogue input , same question for analogue output"
Sounds like he has more faith in the analog sensing device in the field than he has in the Analog Input Module.
Amr...
If possible, I would suggest that you go the extra step and make the whole thing redundant... field devices are just as likely, if not more likely, to fail than the analog-to-digital converter.
I can only assume that you are trying to ensure that you have the analog signal when you need it... which, of course, sounds reasonable. So...
Install two analog sensors and two analog inputs... each installed as individual sets.
You could then install a physical switch to switch from one to the other, as needed... or...
If your software allows, and if it provides the appropriate status signals indicating the condition of the particular analog input, then you could monitor those signals and let the program decide which input to use.
One input would necessarily have to be consigned the role of primary, the other would be consigned to the secondary, or backup, role.
If "Input-A is OK", use Input-A.
Copy Scaled Input Value from Input-A to Vxxx. Use Vxxx in your program.
If NOT "Input-A is OK" and "Input-B is OK", use Input-B.
Copy Scaled Input Value from Input-B to Vxxx. Use Vxxx in your program.
In this case, Input-A is the primary, and Input-B is the secondary.
If NOT "Input-A is OK" -AND- NOT "Input-B is OK", then set an Alarm!
Alternatively, you could... I wouldn't be inclined to do so, but you could...
Develop a "lock-in/lock-out" scheme.
You could use two analog field devices and one analog input through a relay.
One analog field device is wired to the Normally Open contacts of a Double-Pole/Double-Throw relay.
The other analog field device is wired to the Normally Closed contacts of a Double-Pole/Double-Throw relay.
The Common terminals go to the Analog Input Module. (Be aware of polarities!)
Now, assuming that Input-A is the primary...
Possibilities...
Relay Normally OFF (process-wise)... use Input Device "A"
If Relay goes ON, then use Input Device "B".
Relay Normally ON (process-wise)... use Input Device "A"
If Relay goes OFF, then use Input Device "B".
If the Relay is Normally OFF (process-wise)...
When you power-up the PLC the operational status of the field device "A" should be apparent immediately. As long as the operational status of Input-A is OK, then keep the relay OFF. The PLC will continue to use the signal from Input-A.
If, however, the operational status of Input-A comes in negative, that is, you receive a negative status while the relay is OFF, then you simply "lock-in" the alternate source and "lock-out" the primary source. That is, you turn the relay ON and hold it ON. As long as the relay is ON and the operational status of the input is OK then you use the signal from Input-B. You'll need a timer for the transition.
You will not be able to check the status of the primary, because it will be locked-out. So, you will not be able to "go back" to the primary if it is restored, unless you provide a manual means to do so.
If the Relay is Normally ON (process-wise)...
To do it this way, you need to have a timer to allow the relay to close, on power-up, before you check the operational status of the input.
When you power-up the PLC the timer will allow the relay to close before you check the operational status of Input-A. Then, after the timer, if the operational status of Input-A is OK, then keep the relay ON. The PLC will continue to use the signal from Input-A.
If, however, the operational status of Input-A comes in negative, that is, you receive a negative status while the relay is ON, then you simply "lock-out" the primary source and "lock-in" the secondary source. That is, you turn the relay OFF and hold it OFF. As long as the relay is OFF and the operational status of the input is OK then you use the signal from Input-B. You'll need a timer for the transition.
You will not be able to check the staus of the primary, because it will be locked-out. So, you will not be able to "go back" to the primary if it is restored, unless you provide a manual means to do so.
Now... regarding the redundant analog outputs...
Is this a case of two analog outputs to a single analog device? Or a case of one analog output to two devices?
Again, I can only assume that you are trying to ensure that you get the analog output signal when you need it... again, that sounds reasonable.
HOWEVER... right now, MGD is doing what MGD is supposed to do... so, all I can say, right now, is that the output scheme is similar to the input scheme... sorta... kinda... maybe you can figure it out based on the Input example.
Hey... c'mon... it's 12:30 AM, I had an 18-Hour (100-degree +) kinda day...
I start at 4:00 AM every day... and it's 90-degrees at work, everyday, before 8:00 AM... This MGD, and many more, are for ME!
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