1769-L30ER with 1769-IF4XO2 and PID/Scaling

Hey everyone,

Doing a project for a VFD fan controlling the temperature of the condenser at an arena. The VFD replaces a two-motor setup that was originally installed. Nothing has been field tested yet, this is getting the programming done ahead of time.

There is an analog (4-20 mA) pressure transmitter (0-300 psi) and an analog output to the VFD (Siemens V20) for its speed reference (25-100%), setpoint value is 150 psi. I think I have the PID configured correctly for those parameters. I am currently using bits from the PID to display the PSI and speed (as a %), using Condenser_PID.PV and .OUT to get a scaled 0-100% value that I can update the HMI (Panelview 1000) with. The PID does all the scaling, no need for separate scaling or scaling with parameters, correct?

When I verify the routine, I get errors in my MAP_INPUT and MAP_OUTPUT routines, saying "Error: Rung 31, OTE, Operand 0: Invalid data type. Argument must match parameter data type." Should I be mapping the analog signals to a separate memory address, then reading/writing to those for the PID and HMI? In the PID, I used the analog input and output channels directly.

The issue is not how you're mapping anything, it's just data (tag) types. An OTE needs a BOOL tag assigned to it, and you have assigned an INT/DINT/REAL/something else.

ASF said:

The issue is not how you're mapping anything, it's just data (tag) types. An OTE needs a BOOL tag assigned to it, and you have assigned an INT/DINT/REAL/something else.

Click to expand...

An analog Input or Output needs a REAL data type, doesn't it? So is there a way to map that value, for use in the program elsewhere?

The error isn't in your analog input/output, it's in an OTE according to the text you posted... What is the OTE on rung 31 doing, and how do you have it addressed?

LoganB said:

The error isn't in your analog input/output, it's in an OTE according to the text you posted... What is the OTE on rung 31 doing, and how do you have it addressed?

Click to expand...

It was there as part of the mapped inputs and outputs ladder, but I deleted it and the corresponding analog output for the VFD speed reference. It was addressed as Local:5:I.Ch0Data. When I look at the Controller Tags, these are data type INT, with the Data.0-15 being BOOL. I thought they had to be REAL for analog values?

Nope, that depends on the VFD itself and the EDS file. A 1769-IF8 for instance (analog input card) has INTs for each channel value.

If you want to use an OTE for something, you would need to address it at the bit level (Local:5:I.Ch0Data.0 for example). OTEs can only be used to monitor binary addresses.

You would need to use a compare like a GRT (Greater Than) to monitor the value of an analog channel in its entirety.

hopefully this doesn't confuse, some of my numbers might be off.

Local:5:I.Ch0Data is your raw card data and is usually an INT I think only the 1756 line can have it scaled on the card to a real value.

AFAIK the 1769 only allows the following:
• Engineering Units in volts or milliamps. 4000-20000
• Scaled-for-PID.
• Percent of range.
• Raw/Proportional Data for maximum resolution. "This would be in counts and the max value depends on the module being used"


now just to try to demystify lets look at the Raw/Proportional
pretend we have a 4-20mA pressure transmitter ranged from 0-100psi.

at 50 psi the transmitter outputs 12mA when this gets to the plc it needs this in binary form to work with it so the AIN card has an ADC analog to Digital converter to do just this.

in your case the adc resolution is 8bits from 0-20mA, which seems awfully low, so full scale 20mA would be all eight bits on or 1111 1111 which in for clarity sake can be represented as 255 counts in integer format which gives you a value of almost 12.75 counts per mA.

so at 12mA you would see roughly 153 counts this is your plcs way of representing 12mA however it does not know that this 12mA represents 50psi.

this is where you would scale it Local:5:I.Ch0Data to make it human readable
you scale it from 51 count - 255 counts to 0-100psi the 51 is for your 4mA zero since your transmitter outputs 4-20mA but the card reads 0-20mA so 4X12.75 counts/mA = 51 counts. you would want to send it to a real tag to get the read your decimal values however your resolution is ultimately limited to your ADC resolution.


you need to do the opposite for Analog outputs usually convert to the raw umber expected by your module then that go to a DAC digital to analog converter to send out your 4-20mA

oh and those bits 0-15 are just the binary representation of the 16bit INT word

labeledas: thank you for the detailed response! I am using the PID to do any scaling at the moment (pressure input and vfd speed reference output), which is new to me, the last time I did a PID I had to scale with parameters (on an SLC system) for the input/output to be useable by the process.

I've got an understanding of how scaling 4-20 mA works with real world (0-300 PSI in my case) measurements. My confusion is in how the data is addressed internally (INT vs REAL), and then if I need to scale the analog signals to a memory location for use. I think I understand it now...

Hopefully this attachment has the relevant info.