redundant analogue input

please advise how can i connect a two wire transmitter to redundant anlogue input , same question for analogue output

With an input, just wire them all in series, but be sure to keep the polarity correct.

This also assumes that you have verified the loop resistance and it is less than the maximum specified by your 2-wire transmitter device.

As for doing the same with an analog output... I'm not sure that this can be done with a mA output. It may be possible by using voltage outputs wired in parallel, but don't quote me on that one.

jimbo3123 said:

With an input, just wire them all in series, but be sure to keep the polarity correct.

Click to expand...

Many AI are not differential and isolated so you cannot wire in series. If the AI's have external resisters (to from voltage from current) then you can wire in parr. if you remove one resistor.

Otherwise, use a loop isolator / repeater so that the second AI can use the current signal.

Hold on a bit , have I missed something ? he wants to connect a two wire sensor to an AI , not two sensors to one input , is this right ?
If so , the you will need to find a source for your power , some cards have this ready for you , some you have to nick from a +ve common . With siemens , be VERY careful of the selectors on the side of the card , if you select two wire , and use an external power supply , you will smoke the card , in this case , select 4 wire . Effectively anything that is sourcing 20mA needs to be set for 4 wire , anything that sinks 20mA , for two wire .
Siemens cards smoke with regularity because of this !

Amr Hassan,

You haven't given any specifics about your system, so it's impossible to give any specific answer. However, some general comments:

If your I/O hardware is not designed to be used as redundant I/O, you will probably add more failure modes than you eliminate by wiring *single* devices to two I/O points. The inputs can be split to two channels with islolators. The ouputs are much more complicated since you need some method of determining a channel failure and switching outputs and this happens *outside* the I/O system. The point is, all this extra hardware *lowers* reliability.

If you want to improve the reliability of a system that is not designed for redundant I/O, I suggest you use two transmitters wired to channels on separate I/O cards in separate chassis. Totally independant, nothing in common.

Depending on the process and your final control elements, you may be able to do the same with outputs. However, this is usually much more difficult and costly.

Seriously, if a risk analysis indicates that your I/O cards are the weak point in your control system, you probably need a different system.

Good luck,

Mike Ellis

Amr,

Please explain what you mean by "redundant". In some parts of the world this means "extra", "unused", or "spare". If this is what you mean, then just wire it up like the Allen-Bradley Input/Output manuals show.

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!

(107)

Wow Terry !!

I think he is refering to redundant as being "not being used" rather than redundant in a more PLC meaning - however your reply should give him something to read.

Mr. Hasan, your scheme is more appealing intellectually than practically. I would appreciate it if you could provide the reason you want to use redundant I/O.

PLC I/O is generally at least as reliable as field devices, so the increase in reliability is limited.

In my experience Analog cards fail completely (all channels at once) at least as often as they do a single channel at a time. You could apply the system to separate cards, not just separate channels, but now the cost is probably exceeding the benefit.

In my experience the most common cause of analog failure is a voltage spike or transient from lightning, a short circuit, etc. In this case using multipe I/O on a single set of wires means you'll probably smoke two channels at once.

The problem of shared commons on analog cards mentioned above is significant.

Having said that, I have used Terry's relay scheme to use two different analog output signals to a single actuator in the past. In my case it was a local/remote controller selection, with two completely different controllers providing the 4-20 mA signals. You want to use bifurcated gold relay contacts if possible, try to verify "break before make" operation on the contacts, and understand that there will be a very short total loss of signal to the actuator during contact switching.

Alslam Alikom Amr,

Great replies from all, thanks all for the info.

And to my friend Amr, Take a look at This thread.

Best regards.

which one was it , a two wire device to two cards or to an unused card?