Configuring DmD SCALE Output Clamping for Analog Signals

The scaled mA value stays within its selected lower and upper limits after SCALE feeds CLAMP, or after the scaling logic is replaced with LERP. Keep the unclamped reading available for diagnostics so the production value cannot overrun its permitted range without hiding an input fault.

Stop patching SCALE with more comparisons

The usual quick fix adds comparisons, moves, and branches around every scaled analog value. It can restore production, but repeated custom logic creates more places for limits to disagree and more code to inspect during a night-shift fault.

Do not wait for a clamping checkbox in SCALE. The described implementation explicitly does not clamp its actual output. Use one of the instructions already intended for the job:

• Retain SCALE and place CLAMP after it when the existing conversion is correct.
• Replace the scaling and manual limiting logic with LERP when scaling with built-in clamping fits the application.

Do not force the raw input to an endpoint before scaling merely to suppress an excessive engineering value. That destroys the distinction between a field signal outside its reference range and a valid signal sitting exactly at the limit.

Check whether SCALE is extrapolating

Read three values in the same scan: the raw analog value, the direct output from SCALE, and the final value consumed by the control logic. Also record the configured input and output reference endpoints.

A linear scale follows this relationship:

scaled = output_low + (input - input_low)
         * (output_high - output_low)
         / (input_high - input_low)

When the input moves beyond either reference endpoint, the same line continues beyond the corresponding output endpoint. That is extrapolation, not a failed scaling calculation. Because SCALE does not clamp, an out-of-range current reading can produce an out-of-range engineering value.

If the raw value lies between its reference endpoints but the result is wrong, stop adding clamps. Check endpoint order, endpoint values, the source register feeding the instruction, and the destination used by downstream logic. A clamp can conceal a conversion or addressing error while leaving the process calculation wrong inside the range.

Choose the value that must be limited

Separate measurement evidence from the value allowed to influence control. Retain the raw input and, when useful, the direct scaled result for alarms and maintenance. Send only the clamped result to logic that must stay within defined engineering limits.

A proposed change to SCALE could make lower and upper clamping optional, but that is not the current resolving path described here. Build against the instruction behavior present in the installed programming environment.

Follow the readings through the decision tree

Stop here if the unclamped value could command unsafe motion, heating, pressure, or another hazardous response. Isolate that command path according to the machine procedure before forcing or simulating analog values.

Apply the production fix

1. Record the current input references, output references, raw reading, direct scaled result, and required control limits.
2. Preserve the raw input in the diagnostic path. Keep the direct SCALE output visible if maintenance needs to identify extrapolation.
3. For the smallest logic change, connect the SCALE result to CLAMP. Configure the required lower and upper output limits in the clamp instruction.
4. For integrated scaling and clamping, replace the scale and manual limiting statements with LERP. Enter the same validated reference relationship and configure its clamping behavior through the instruction editor.
5. Redirect control calculations, indications that require a bounded value, and commands to the final clamped result. Leave diagnostic alarms connected to the appropriate raw or pre-clamp value.
6. Search the program for other consumers of the old destination. A correct clamp does nothing for logic that still reads the unclamped tag or register.

Diagnostic path: raw input -> SCALE result
Control path:    raw input -> SCALE -> CLAMP -> consumer
Alternative:     raw input -> LERP -> consumer

Verify both limits and the fault evidence

Test with an approved simulator, source, or controlled process condition. Do not judge the repair from one normal operating point.

1. Apply a value between the input references. Confirm the final value follows the expected linear conversion and is not held at a limit.
2. Move toward the lower reference, then beyond it. Confirm the diagnostic value can show the excursion while the control-facing value stops at the selected lower limit.
3. Repeat the test at and beyond the upper reference. Confirm the control-facing value stops at the selected upper limit.
4. Return to the valid span. Confirm the final value leaves the limit and resumes tracking without requiring a reset.
5. Check alarms, trends, interlocks, displays, and control calculations separately. Confirm each one uses either the raw, scaled, or clamped value by design.

If a boundary differs by a small numeric increment, inspect the instruction data types and displayed precision before changing the limits. If the conversion is discontinuous, uses identical input reference points, or changes in the wrong direction, correct the reference configuration instead of masking it with a wider clamp.

FAQ