Troubleshooting MICROMASTER 440 Analog Output No mA at Terminals

Problem Overview

The Siemens MICROMASTER 440 (MM440) inverter is widely deployed for variable-speed control of three-phase motors in HVAC, conveyor, pump, and process-line applications. A common field failure reported by maintenance engineers is a complete loss of milliampere signal on the analog outputs while the drive itself continues to run normally and the parameter readout r0774 still updates on the BOP/AOP. The reported symptom: terminals 12-13 and 26-27 source zero current even though the drive is in RUN, the setpoint is being followed, and the smoothed DAC value is being computed correctly inside the drive.

Because the analog output on MM440 is generated by a dedicated DAC (Digital-to-Analog Converter) on the control board located beneath the Basic Operator Panel (BOP), this symptom almost always points to either a hardware fault on the I/O card, an open-loop wiring path, an instrumentation-side burden impedance problem, or a parameter scaling issue. The remainder of this article walks through the diagnostic decision tree using the actual case parameters:

• P0771 = 21 (signal source = actual frequency, scaled)
• P0773 = 2 (output range = 0-20 mA)
• P0775 = 0, P0776 = 0, P0777 = 0, P0778 = 0 (default value mapping)
• P0779 = -100, P0780 = -20 (output characteristic endpoints)
• r0774 = 8.75 % at 20 Hz operation

Affected Hardware and Terminal Layout

The MICROMASTER 440 control board has two configurable analog outputs. Their terminal assignments and electrical characteristics are summarized in the table below; refer to the MM440 Parameter List and Operating Instructions for the complete pin map of the control block (the 30-pin removable connector at the top of the I/O card).

The DAC for both outputs is implemented on the same I/O card that sits underneath the BOP. This is the printed circuit board that carries the removable control-terminal connector and the on-board MCU. The card is field-replaceable as a subassembly; the surrounding chassis, power stack, and operator panel are separate spares. When the DAC fails (truly dead output, not scaling, not a wiring problem), the entire I/O PCB must be replaced and recommissioned.

Parameter Configuration Analysis

The reported parameter set is internally consistent. P0771 = 21 ties the analog output to the actual frequency (Hz) of the drive as a percentage of P2000; P0773 = 2 selects 0-20 mA current mode; and the characteristic endpoints P0779 = -100, P0780 = -20 map the -100 % to -20 % region of the source signal onto the 0-20 mA output. Because r0774 reports the post-scaling, post-filter value of the DAC drive word, an r0774 = 8.75 at 20 Hz operation implies the firmware is computing a valid output demand. The fault, therefore, is downstream of the firmware.

The smoothing time P0772 implements a PT1 filter on the DAC output. Per the official parameter list, the smoothing time is defined in milliseconds and accepts a positive absolute value only. For high-bandwidth feedback (e.g. closing a tight PID loop on a PLC), set this to a small value; for noisy speed references, increase it. Documented details are in the MICROMASTER 440 Parameter List (Siemens Support, document 24291961).

Root Cause Analysis

When the firmware-side DAC word updates correctly (as proven by r0774) but no milliampere signal is measurable at the terminals, the chain of suspects is short and ordered by likelihood:

1. Open current loop — terminals 12 (or 26) and 13 (or 27) are not connected to a return path. An MM440 current output is an active current source; it will only drive current into a closed circuit with a burden of 0-500 Ω. If the loop is broken, the DAC still attempts to source 20 mA but the current simply cannot flow.
2. Burden impedance below the DMM shunt — a digital multimeter placed in current mode on the low mA range presents a very low shunt (often less than 50 Ω). Combined with any other series resistance, this can pull the loop below the DAC compliance voltage and prevent current regulation.
3. Incorrect range selection — P0773 may have been changed by an inadvertent download of parameters from a starter file. A P0773 = 0 or 1 setting routes the DAC to voltage output mode where terminals 12-13 and 26-27 become 0-10 V outputs, not 0-20 mA.
4. DAC hardware failure — the on-board DAC or its output driver transistor is damaged (over-voltage, surge on the analog loop, or thermal stress). The output is truly dead; the firmware drive word is correct but no current reaches the terminals.
5. Corroded or loose removable control-terminal connector — the I/O card uses a screw or push-in terminal block that can work loose in high-vibration installations. Contact resistance rises to the point where the loop is effectively open.

Diagnostic Procedure

1. Verify drive state. Confirm the MM440 is in RUN (not OFF or in fault). Note the motor speed on the BOP.
2. Read r0774 live. Toggle RUN on/off and watch r0774. At standstill, r0774 should reflect the lower endpoint of your characteristic. During run, it should scale with frequency.
3. Re-confirm P0773. Navigate to P0773 and confirm it is 2 (0-20 mA). If it has been changed to 0 or 1, you will be looking at a voltage output and the DMM in current mode will read 0 mA — not because the output is dead, but because the loop is configured to source voltage instead of current.
4. Inspect wiring. Open the control terminal cover and visually inspect the terminal block. Re-torque the screws on terminals 12, 13, 26, and 27 to the published torque (typically 0.22-0.25 N·m for the MM440 control terminals).
5. Insert a known burden. Disconnect the existing load (PLC analog input, indicator, recorder) and connect a precision 250 Ω, 0.1 %, 1 W resistor across terminals 12-13 and 26-27. Use a separate DMM in voltage mode across the resistor; you should read between 0 and 5 V DC corresponding to 0-20 mA. This decouples the measurement from the DMM shunt and from the PLC's analog input impedance.
6. Measure with DMM on a current range with adequate burden. If you must use the DMM directly, place it on the 20 mA or 200 mA range (not the autorange if your DMM autoranges through a low-shunt 2 mA range first). Confirm the DMM shunt resistance is below 500 Ω but well above 0; 100-250 Ω is typical for a 20 mA range.
7. Swap output channels. If the value P0771 is reconfigurable to source the same signal, route it to analog output 2 and check terminals 26-27. If output 2 works but output 1 does not, the failure is in the DAC channel-1 output stage, not the parameter set.
8. Power cycle and re-test. Power the drive off, wait for the DC bus to discharge (the BOP/AOP display will go blank), then re-apply control power. Some DAC latch-up faults recover with a hard reset; many do not.
9. Replace the I/O card. If all the above yield zero mA on at least one output and the firmware is updating r0774, the I/O card (the PCB below the BOP) must be replaced. Reorder using the MLFB printed on the existing card label and cross-reference with the Siemens MM440 spare parts list for the specific frame size (FS-A through FS-F).

PLC Interface Considerations

If the analog output is being read by a PLC analog input module, the symptom can also originate on the PLC side. Before condemning the drive's I/O card, verify the following on the PLC:

• The analog input channel is configured for current mode, not voltage. S7-200/S7-1200 SM modules and most third-party PLCs have a hardware jumper or DIP switch that selects voltage vs current.
• The PLC input channel is enabled in the hardware configuration (TIA Portal / STEP 7 / RSLogix).
• The scaling block in the PLC program matches the drive's output characteristic. A common error is to scale a 4-20 mA input assuming 0-20 mA, which yields a permanent -25 % offset.
• Wiring between the drive and PLC maintains polarity. The MM440 sources current; the PLC input must be wired so the current enters the positive terminal and returns to the drive's signal common. Reversed polarity on a current loop is functionally an open circuit.

For a closed-loop process where the PLC PID is reading the drive's output frequency via the analog output, the loop bandwidth must be higher than the inverse of P0772. A 200 ms smoothing time on the DAC, for example, introduces 200 ms of dead time that will destabilize any fast PLC-side loop.

Solution: I/O Card Replacement

The field-proven corrective action when the DAC hardware is dead is to replace the I/O PCB assembly located beneath the BOP. The procedure is:

1. Lock out and tag out the drive. Wait five minutes for the DC bus to discharge below 50 V DC.
2. Remove the BOP by pressing the two side release tabs; the BOP will hinge forward and disconnect from its ribbon header.
3. Remove the control terminal connector block (the green removable plug at the top of the I/O card).
4. Remove the four mounting screws that secure the I/O PCB to the chassis. Note the position of any insulating washers or stand-offs.
5. Lift the I/O card straight out. Do not flex the board; the rear edge has a connector that mates with the power stack controller card.
6. Install the replacement card in the reverse order. Confirm the rear connector is fully seated and the BOP ribbon is not pinched.
7. Re-apply control power and download the parameter set from the operator panel or a STARTER commissioning file. Do not assume the new card ships with the same parameter set — the MM440 stores parameters on the control card, so a swapped card may default to factory parameters.
8. Re-enter or restore P0771 = 21, P0773 = 2, P0775-P0778 = 0, P0779 = -100, P0780 = -20, and any other application-specific parameters.
9. Verify the analog output with a 250 Ω burden before reconnecting the PLC load.

Verification

After replacement, perform the following acceptance test:

1. With the drive in OFF, confirm the analog output sits at the lower endpoint: with P0779 = -100 and P0780 = -20, the output should be 0 mA at standstill when the source signal is at -100 %.
2. Issue a RUN command with a 50 Hz setpoint. Confirm r0774 tracks the expected scaled value (5.0 % at 50 Hz if P2000 = 50 Hz).
3. Measure the analog output with a calibrated DMM on the 20 mA range with a 250 Ω shunt. The reading should be monotonic with speed and within the published accuracy (typically ±1 % of full scale).
4. Reconnect the PLC analog input and confirm the engineering units in the PLC match the expected range.
5. Run the drive through its full speed range and log both r0774 and the PLC input value. The two curves should overlay when scaled.

Preventive Maintenance and Long-Term Reliability

• Specify shielded, twisted-pair cable for the analog loop. Ground the shield at the cabinet end only to avoid ground loops through the shield.
• Add surge protection on the analog output pairs if the cable leaves the cabinet. The MM440 has limited built-in transient suppression on its analog I/O.
• Periodically (annual) re-torque the control terminal screws; vibration and thermal cycling can loosen push-in connectors.
• Keep the BOP connector clean. Dust and humidity on the BOP ribbon header are a common cause of intermittent I/O faults that mimic DAC failure.
• Maintain a parameter backup on the AOP or a STARTER project file. The MM440 stores parameters on the control card; replacing the card without a backup forces a full recommissioning.

Related Faults and Edge Cases

A few failure modes present identically to a dead DAC and should be ruled out before a card swap:

• Drive in fault state. Some MM440 faults latch the analog outputs to 0 mA even if the DAC itself is healthy. Check r0947 for the active fault code and clear before re-measuring.
• Safety enable missing. If the safety input (terminals 31-34 depending on variant) is not asserted, the drive will not run and the analog outputs will reflect 0 speed, not a hardware failure.
• Controller in inhibit with P0771 mapped to output frequency — the output will be 0 mA until a RUN command is issued. Test the output with a setpoint that does not depend on the actual motor running, e.g. map P0771 = 22 (ramp input) or P0771 = 24 (output current).
• DC injection braking active — when DC injection is enabled and the drive is stopped, the analog output is held at the lower endpoint until the injection phase ends. This is normal, not a fault.

Frequently Asked Questions