Productivity 2000: How do I scale a 4-20 mA input?

A 4–20 mA level transmitter on a Productivity 2000 input needs an SCL input minimum equal to the raw count at 4 mA, not zero; then map that corrected span to the tank units sent to the C-more panel.

Read the analog channel before editing SCL

Monitor the input channel tag online while the transmitter is connected and the process is in a known state. Record the raw count at the low end and at a second known level if available. A current input scaled internally for 0–20 mA will show a nonzero count at 4 mA; that is expected and does not mean the transmitter is mis-zeroed. The SCL instruction must remove that offset by treating the 4 mA count as its input minimum.

Use this first reading to choose a branch:

• If the raw value changes as the measured level changes, continue to identify the module resolution and calculate the scale endpoints.
• If the value stays fixed, moves in the wrong direction, or is clearly outside the expected range, check module channel configuration, loop wiring, transmitter power, and actual loop current before changing scale values.
• If the PLC raw value responds correctly but the C-more display does not, finish the PLC scaling check, then inspect the panel’s tag binding and communications.

Do not fix an incorrect reading by changing the output range. The output range defines the displayed units; it cannot correct a channel that is configured for voltage, wired incorrectly, or using the wrong raw-count resolution.

Identify the module resolution and raw-count endpoints

Match the installed input module and channel configuration to the correct raw-count representation. The examples below use the two 0–20 mA spans given for Productivity 2000 inputs; they are not interchangeable.

Input representation	0 mA count	20 mA count	Calculated 4 mA count
16-bit module example	0	65535	13107 (nearest whole count)
Low-resolution 13-bit module example	0	8191	1638 (nearest whole count)

Calculate the 4 mA point by multiplying the 20 mA count by 4/20. With the stated endpoints, the calculations are 65535 × 4/20 = 13107 and 8191 × 4/20 = 1638.2, rounded to 1638 counts. Approximate values of 13104 and 1636 do not follow from those endpoint formulas. Use the actual raw range specified for the installed module if its documented endpoints differ from these examples.

Read the module configuration and channel tag in the PLC project, then compare them with the module’s documentation. If the module’s resolution or current range cannot be identified from the project and its documentation, stop before entering scale endpoints; a plausible-looking output can still be wrong when the raw span is mismatched.

Set the 4 mA zero correctly for this channel

Configure the SCL instruction with the raw count at 4 mA as IN Min and the raw count at 20 mA as IN Max. Set Out Min and Out Max to the engineering range the operator needs. For a 0–100 percent display, use output endpoints 0 and 100. For a vessel calibrated from 50 to 5000 gallons, use 50 and 5000. These output values come from the process calibration and vessel capacity, not from the input module resolution.

Scaled output = Out Min + (Raw input - IN Min) × (Out Max - Out Min) / (IN Max - IN Min)

For the 16-bit example, a nominal 4 mA input is approximately 13107 counts and 20 mA is 65535. For the 13-bit example, those endpoints are approximately 1638 and 8191. The PLC instruction maps those endpoints to the chosen output endpoints, so the corrected output can start at zero even though the raw input is not zero at 4 mA.

Do not set IN Min to zero simply because the input card’s range begins at 0 mA. That would map 4 mA to roughly 20 percent of the output span instead of to the configured low endpoint. Also avoid entering a 16-bit minimum for a 13-bit channel, or the reverse; the scale will compress or distort the usable measurement range.

Choose SCL or SCLN from vessel geometry

Use SCL when level maps linearly to the desired engineering quantity. A straight-sided vessel with a constant cross-section has a linear level-to-volume relationship. The same approach works when the panel only needs to show percent of calibrated level, regardless of vessel shape.

Use SCLN when the vessel geometry makes the level-to-volume relationship nonlinear, such as a vessel with conic sections. Vessel orientation matters: a horizontal cylindrical tank, for example, does not have a constant cross-section over its height. Check the Productivity programming help for the instruction’s nonlinear scaling setup and enter points that represent the actual vessel calibration. Do not select a nonlinear scale merely because the sensor itself is a level sensor; choose based on the relationship between level and the displayed quantity.

Branch on the required display:

• For raw level, percent level, or a straight-sided tank’s volume, define the calibrated endpoints and use SCL.
• For volume in a vessel whose cross-section changes with height, use the appropriate nonlinear mapping and validate it against known fill levels.

Diagnose bad readings before changing scale values

Compare the physical process, raw input tag, scaled PLC result, and C-more value in that order. This separates a loop problem from a scaling problem and a display binding problem.

Symptom	Reading or check	Likely cause and next action
Raw value is zero or fixed while level changes	Check the module channel setup, live loop current, wiring, and transmitter power.	Resolve input configuration or loop operation before changing SCL endpoints.
Raw value changes, but scaled output is offset	Compare the configured IN Min and IN Max with the module’s raw span.	Correct the 4 mA minimum, 20 mA maximum, or resolution selection.
PLC scaled value is correct, but C-more is wrong or frozen	Compare the panel’s selected tag with the live scaled PLC tag.	Correct the panel binding or communications. Avoid scaling the value again in the panel.
Level is accurate but calculated volume is wrong at intermediate fills	Compare indicated volume with vessel calibration at several levels.	Use a nonlinear mapping when the vessel geometry makes volume nonlinear with level.

For a suspicious raw value, confirm the actual current with an appropriate loop measurement method and check the module wiring diagram for the installed channel. Current-output wiring and module input requirements vary by device; do not infer terminal assignments or loop-power needs from another module. If the current is correct but the raw count is not, return to the configured input range and documented count endpoints.

Map the scaled PLC value into C-more

Expose the SCL or SCLN result as a PLC tag with a name that identifies its engineering meaning, then bind the C-more numeric display to that tag. Configure the panel’s units and decimal display to match the output scale—for example, percent or gallons. The panel should show the already-scaled PLC result when PLC logic performs the scaling.

Check the value at both ends: monitor the scaled tag in the PLC and read the corresponding C-more object. If the PLC tag is correct while the panel differs, inspect which tag the object reads and whether the panel applies an additional scale or offset. If both values agree but the number is physically wrong, recheck the input endpoints, output endpoints, and vessel mapping.

Apply the corrected scale and verify the operating span

After confirming the current channel range, raw endpoints, and required display units, use this commissioning sequence:

1. Record the existing input configuration and scaling values so they can be restored if needed.
2. Confirm that the channel is configured for the transmitter’s 4–20 mA signal and that its raw count changes with the process.
3. Select the matching 16-bit or 13-bit count span, or use the installed module’s documented endpoints. Calculate the 4 mA minimum from the actual 20 mA count.
4. Set IN Min and IN Max for 4 mA and 20 mA. Enter the calibrated engineering endpoints as Out Min and Out Max.
5. Select SCL for a linear relationship or SCLN for a nonlinear vessel relationship, then connect the result to the intended PLC tag.
6. Bind the C-more display to that scaled tag and set the appropriate unit and display precision.
7. Verify at a known low point, a middle point, and a known high point. Compare the transmitter or reference measurement, raw channel value, scaled PLC value, and C-more display.

At 4 mA, the output should match Out Min; at 20 mA, it should match Out Max, subject to the installed module’s raw resolution and calibration. Check at least one intermediate point to expose a wrong scale span or vessel curve. A correct endpoint match alone does not prove that a nonlinear volume display is accurate through the vessel’s operating range.

Productivity 2000 analog input scaling FAQ

Stop commissioning if the installed module’s current range, raw representation, or wiring cannot be identified from its documentation, or if calibrated checks do not agree with the scaled value. Contact AutomationDirect technical support with the module configuration, raw readings, scale endpoints, and vessel calibration data.