Set an HMI setpoint that drives a PLC analog output

What you need

• The analog output module's documentation page for its configured range: signal type (voltage or current), the range itself, and the raw minimum and maximum counts the module accepts for that range. Every number in the scale rung comes from that page.
• The receiving controller's input specification: its signal type and range. The two devices must be configured for the same signal type; a correct count value cannot rescue a current loop wired to a voltage input.
• Access to the HMI project and to the PLC program.
• A meter that reads milliamps, connected at the loop test jacks or in series.
• A test window with the loop in manual, so the span points can be driven safely.

Give the setpoint its own register

Pick a register that serves only the operator's request, and keep the output image address off the screen. Keeping the command separate from the output lets the PLC validate and scale the value before any of it reaches hardware.

Configure the numeric entry object on that register, label it in PSI, and set its entry limits to the process span where the object supports limits. The entry limits are the first filter; setpoint entry limits configured on the HMI shows how to make the object itself refuse out-of-span values. The PLC clamp comes later as the second filter, so a value that arrives by any other write path still gets held.

Test the write path before touching the scale: watch the register while you type a value on the keypad, the same check operator panel link setup: test writes before motion runs before machinery moves. If the register takes the value and holds it, the write path works. If it changes briefly and reverts, another rung or screen object writes the same register, and that writer has to be found before the scale will mean anything.

Clamp and scale in the PLC

Two rungs do the conversion. The clamp holds the setpoint inside the span: below 0 becomes 0, above the span top it holds the span top. Clamping HMI numeric entry limits in PLC ladder logic builds that rung step by step. The scale rung then converts the clamped PSI to raw counts:

counts = RawMin + (PSI / span) x (RawMax - RawMin)

RawMin and RawMax are the documented count endpoints for the module's configured range, and span is the process span in PSI. Keep both rungs in the PLC even with entry limits on the object: the clamp has to hold for a value that arrives without the keypad.

Use one numeric type from register to output and let the PLC's documented rounding apply once. A mid-span entry produces a half-count result in some integer widths, and one rounding at the end is easier to predict than rounding in two places.

Example: scaling a 0-200 PSI setpoint into a 4-20 mA output

Inputs, and where each comes from:

• Process span 0 to 200 PSI: the scale this loop is built to, 0 PSI at loop zero and 200 PSI at full span.
• Output signal 4-20 mA: the current span the module is configured for and the receiver expects, a 16 mA span above the 4 mA zero.
• Raw range 0 to 27648 counts: the documented nominal span several 16-bit analog module families use for a configured current range. The module manual prints its own endpoints; this example uses that printed value.
• Receiver input: the current signal mapped over the same process span.

The rule in words: counts = 0 + (PSI / 200) x (27648 - 0), after the clamp.

The arithmetic:

• Scale factor: 27648 / 200 = 138.24 counts per PSI.
• 100 PSI: 138.24 x 100 = 13824 counts. Loop current: 4 mA + (13824 / 27648) x 16 mA = 4 mA + 8 mA = 12 mA.
• 0 PSI: 0 counts, 4 mA. Full span: 27648 counts, 20 mA.
• Resolution: 200 / 27648 = 0.0072 PSI per count, so the setpoint moves in fine steps.

The decision: at these three entries the module must hold 0, 13824 and 27648 counts and the meter must read the matching 4, 12 and full-span currents. If the counts are right and the current is wrong, the fault sits in the channel configuration, the wiring or the signal type. If the current is right and the receiver shows a different pressure, set the receiver's input to the current span over the same process range and the three points line up.

Prove the path before handback

1. Put the loop in manual or the state your plant uses for a controlled output test, and note the last good setpoint. Drive a live output only when the process can accept the span points.
2. Enter 0 PSI. The register reads 0, the module holds 0 counts, the meter reads 4 mA, the receiver displays 0 PSI.
3. Enter 100 PSI. Register 100, counts 13824, meter 12 mA, receiver 100 PSI.
4. Enter 200 PSI. Register 200, counts 27648, meter 20 mA, receiver 200 PSI.
5. Try the clamp: type 250. The entry object refuses it; if a second write path puts 250 in the register anyway, the clamp rung holds the counts at 27648.
6. Restore the noted setpoint and return the loop to automatic.

If a check fails

Work the list in order; each symptom points at one device:

• The register never takes the typed value: the write path. Check the object's register address, the communications status, and whether a rung overwrites the register every scan.
• The register takes the value but the counts stay put: the scale rung is not executing, or it reads a different register than the entry writes.
• The counts are right and the current is wrong: channel configuration, wiring or signal type.
• The current is wrong by a constant factor, such as ten times or half: the span endpoints in the scale rung are swapped, doubled or omitted. The value ten times too small is the most common shape of that error.
• The current is right and the receiver is wrong: the receiver's input range; set it to match the current span.
• The output sits at one value no matter the entry: the channel or the module, and the stuck analog output checklist works through it.
• 0 PSI reads a fixed amount above 4 mA: an offset left in the endpoints, and removing an offset using PLC raw counts clears it.

Recommendation

Recommendation: keep the conversion in the PLC and the operator's value in PSI for any linear setpoint driving a linear analog output. Set the entry object's limits to the process span as the first filter, keep the clamp rung as the second, and prove the three span points at the module and the receiver before the loop goes back on control. If the process variable is nonlinear against the signal, such as liquid volume against level in a shaped tank, leave that conversion out of this path and run it as its own calculation; analog scaling is linear only.

If the cell also needs a crew-facing board that shows the active setpoint and line status, that is a separate shop-floor display job; our shop-floor signage covers it.

Questions