1756-IF16 Analog Input Module/Configuration

jtashaffer

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Join Date
Aug 2009
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KY
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I have a 1756-IF16 Analog Input Module. It is setup for Differential Mode. I am trying to read the 4ma to 20ma signal from a flow meter. I am using a SCP instruction.Range is 4mA=0 and 20mA=162. I have it wired to Channel 4. The positive is wired to IN-8 and The negative is wired to IN-9 with the jumper to iRTN-8. When I had the jumper installed it would give me a negative input data of -40.48728. The output data after scaling is -40.909935. When I remove the jumper the input data goes positive to 167.89609. The output data after scaling is -38.800053. I cant make the data match the data on the flow meter. Any Ideas?
 
Add a 250 Ohm precision resistor across the input. Leave the 8-9 jumper installed.
Edit: Switch polarity of the 4-20mA signal too.
 
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The jumper between IN-9 and iRTN-8 inserts the resistor into the circuit so no external resistor is required. The input (-40.48728) you are getting with the jumper installed indicates an input of zero milliamps. Have you tested the transmitter output to be sure it is actually putting out a signal?

Also be sure channel 4 is configured for 0 - 20 mA input.
 
It is configured to 0 to 20 mA. As far as the flow meter signal I haven't tried it yet. danny.michael the jumper needs to be there?
 
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Check your card scaling for that channel. Make sure you're scaling just one time, either in the card configuration where 0=0 and 20=20 and then scale it or just use the card scaling with your 4=0 and 20=162.
 
It is configured to 0 to 20 mA. As far as the flow meter signal I haven't tried it yet. danny.michael the jumper needs to be there?

Yes, the jumper between IN-9 and iRET-8 needs to be there. Is this a 2-wire transmitter? If so you will need to insert a power supply in the loop.
 
I don't see where this piece of the puzzle has been mentioned - but whatever else you do - put a milliammeter in series with the module ... make sure that the module is actually seeing the field signal that you THINK its seeing ...
 
Let's back up a bit. What type flow meter is this? Manufacturer and model number would be helpful. You say the input data doesn't match what you see on the meter. So the meter is powered up and you see a flow rate on the meter display? If so, have you measured the output current with a milliamp meter? Disconnect the wires going to the PLC and put your meter right across the output terminals on the transmitter. What do you see there?
 
It is a gas flow meter made by Thermal Instrument Company. The meter is powered up. The model number is attached. I will have to wait till I get back to work to use a meter to test it.

image.jpg
 
The scaling is setup the way I mentioned. Power supply in the loop?

I don't see exactly where you say what the card itself is scaled for? Not the SCP, but the input card setup.

Also- from the documentation, it appears the loop is self powered from the device so you shouldn't need to connect a 24dc power supply.
 
Correct it is self powered. I am not for sure if I need to scale it or not or how it needs to be wired up. With the jumper on the analog card I get a negative number with out the jumper I get a positive number but it doesn't match the pph number on the meter. I have to subtract the number to match. If I failed to mention the meter is factory calibrated. Also it doesn't float if I am not using the SCP instruction.
 
Let's work on getting an input from the transmitter into the PLC first. From the manual it looks like the internal protection for the 4-20mA output may be tripped. See below. If you can get a signal, any signal, into the PLC then we can work on scaling. I don't see the need for the SCP since you can scale to engineering units on the IF16, assuming the signal from the transmitter is linear.

Output Signals
Terminals are provided for local instrument powered two-wire signals (positive and
common). If the Temperature Transmission option has been provided, a single wire
common connection may be used or two two-wire cables may be run. There are protective
PolySwitch breakers on all output signal lines and they will “open” up if overloaded but
circuits can still be damaged. When power is on if there is zero output current (there should
be at least four mA even through a current meter) disconnect the field wiring and check the
terminal block points for a voltage. Presence of a voltage (typically 20 to 30 volts) with no
current flow indicates that the PolySwitch breakers have been tripped. Power must be
turned off if the PolySwitch breakers are tripped in order for them to reset. They do not
need to be replaced as a fuse. Maximum load resistance is 500 ohms.
 
ok thanks for all your help danny.michael. I will test it and reset the power if needed on Friday when I get back to work. That makes since about the scaling I am new to programming analog signals.
 
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