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Old May 4th, 2002, 11:30 PM   #1
Eric Nelson
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Question AB Analog Input Module Wiring...

I have a question regarding the 1769-IF4 Analog Input Module (for the ML1500 PLCs). What is the purpose of the "ANLG COM" terminals? The manual states:

All module commons (ANLG COM) are connected in the analog
module. The analog common (ANLG COM) is not connected to
earth ground inside the module.

Are they connected to ANYTHING within the module? Again, quoting the manual:

Voltages on Vin+, V/Iin-, and Iin+ of the 1769-IF4 module must
be within 10V dc of analog common.

This leads me to believe that the "ANLG COM" terminals connect to something, though the manual is unclear as to just "what" that something is...

It looks to me (from the wiring examples), that they're just used as a common connection for the power supply common. Any ideas? I'm just curious... -Eric
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Old May 5th, 2002, 09:31 AM   #2
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Eric, It appears to be just a dummy terminal placed there just for convenience. Depending on how you are wiring your module, (single ended or differential) it may or may not be used. On single ended applications the signal gets wired to the in(+) terminal and a jumper is placed from the in(-) to ANLG Com. It would be my guess that all ANLG Coms are internally connected. It would also appear to me that this is done to insure a zero reference from the common power supply to all inputs.

Ron P.
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Old May 5th, 2002, 10:48 AM   #3
Peter Nachtwey
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>Eric, It appears to be just a dummy terminal placed there just for >convenience.


> All module commons (ANLG COM) are connected in the analog
>module. The analog common (ANLG COM) is not connected to
>earth ground inside the module.

>Are they connected to ANYTHING within the module?

Yes, the 0 volt pin of the differential input chip.

To paraphrase Terry. BE THE DESIGNER!

Given that there is 24 volt power supply to the analog module, one must first convert this to +/- 12 or +/- 15 volts using a DC to DC power supply. The supply side of this DC to DC power supply has three pins. +V, Common and -V. Note that this Common is not connected to the same common as the 24 volt supply. This allows the analog circuitry to float ( it is isolated ). This keeps noise on the 24V supply out of the analog circuitry. When an analog device is connected to the differential input one must make sure the analog signals do not exceed +V or -V or the differential input chip will be damaged. The Common is the 0 volt refrence for the analog circuitry and is provide externally so the extenal analog devices do not return signals that float outside of +V and -V.

This is not a ground. Do not connect it to ground or the analog inputs will be noisy and isolation will be lost.

BTW, usually when designing circuitry that handles +/- 10 volts one needs a +/- 12 volt supply. A little voltage range is lost in biasing and protection circuitry.

All the analong inputs will share the same common unless each input is isolated from the other. This is very expensive.
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Old May 6th, 2002, 08:37 AM   #4
Tom Jenkins
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Peter is absoutely right. These are not "dummy" terminals! In my experience the ML-1500 AI are more sensitive to noise than most PLC inputs, and you have to follow the A-B wiring instructions rigoroulsy.

Helpful tip: I had a 1500 system that worked fine until I tied a kW signal to the analog input card. I developed a tremendous noise problem. This was solved, at the suggestion of an engineer on site, by connecting a 10 k Ohm resistor between "+" and Analog Common on each signal.
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