1. Module Identification and Electrical Specifications
The Siemens SM332 analog output module with order number 6ES7332-5HD01-0AB0 is an 8-channel, 12-bit-plus-sign output card used in the S7-300 family (ET 200M-compatible). It is configured in HW Config (STEP 7 V5.x) or in the Device Configuration (TIA Portal) to deliver one of several standard ranges per channel: ±10 V, 0 to 10 V, 1 to 5 V, ±20 mA, 0 to 20 mA, or 4 to 20 mA. Per the S7-300 Module Data manual, the salient electrical ratings for the 4-20 mA current outputs are:
| Parameter | Value |
|---|---|
| Number of outputs | 8 |
| Resolution | 12 bit + sign (11 bit + sign bipolar / 12 bit unipolar) |
| Output range, current | ±20 mA, 0 to 20 mA, 4 to 20 mA |
| Maximum loop load R_L | 500 Ω |
| Typical loop load | 250 Ω |
| Open-circuit voltage (current output) | ≤ 18 V |
| Accuracy at 25 °C | ±0.5 % of full-scale value |
| Galvanic isolation | Yes, optical, between backplane and outputs |
| Diagnostic interrupt | Configurable (wire break for 4-20 mA) |
| Channel-to-channel isolation | None (channels share common ground) |
| Update time (8 channels active) | typ. 0.8 ms per channel |
Reference: S7-300 Module Data Manual (entry ID 8859629) and the device-specific SM332 (6ES7332-5HD01-0AB0) manual entry. Always verify the firmware version of your IM/CPU and the HSP (Hardware Support Package) installed in TIA Portal before relying on the listed diagnostic features.
2. Why the Output Range Must Be Configured in STEP 7 / TIA Portal
The 6ES7332-5HD01-0AB0 does not auto-detect the output range. Each of the eight channels must be parameterized in the hardware configuration. The default is voltage ±10 V. To obtain 4-20 mA:
- Open the S7 project in STEP 7 V5.5/SPx or TIA Portal (V15 or later recommended for full 5HD01 support).
- Navigate to the SM332 slot in HW Config / Device View.
- Double-click the module and select the Outputs tab.
- Set Output type to Current and Output range to 4 to 20 mA.
- Repeat for each active channel (group of two is typical for 4-channel groups, but the 5HD01 is grouped channel-by-channel per output pair).
- Enable Diagnostics → Wire break if you want a group error LED and OB82 interrupt on open loop.
- Compile and download the hardware configuration to the CPU.
Until step 7 is performed, the analog value written from the user program (e.g., PQW 288) will be scaled to the default voltage range and the output stage will source voltage, not current. A mA reading of zero on a properly wired 250 Ω load is therefore not a hardware fault — it is a configuration mismatch.
3. Loop Compliance and Load Resistor Sizing
The 4-20 mA output of the SM332 is an active current source. The voltage developed across the loop is governed by:
V_loop = I_out × R_load + V_wire_drop
At 20 mA and a 250 Ω load, V_loop = 5.0 V. At 500 Ω, V_loop = 10.0 V. The module can source up to approximately 18 V open-circuit, which gives the headroom needed to drive the maximum specified 500 Ω load.
Verification calculation for the chosen resistor:
| R_load (Ω) | V at 4 mA | V at 12 mA | V at 20 mA | Headroom remaining (V) |
|---|---|---|---|---|
| 100 | 0.40 V | 1.20 V | 2.00 V | 16.00 |
| 250 (typical) | 1.00 V | 3.00 V | 5.00 V | 13.00 |
| 500 (max) | 2.00 V | 6.00 V | 10.00 V | 8.00 |
Lower resistance improves transient response but increases power dissipation in the resistor (0.40 W at 250 Ω / 20 mA, 1.00 W at 500 Ω / 20 mA). For a verification bench setup, a 250 Ω, 0.5 W metal-film resistor is the recommended choice.
4. Required Test Equipment
- Precision ammeter (or DMM in mA DC mode), 0.1 % accuracy or better, 0-25 mA range.
- Load resistor: 250 Ω, 0.5 W, 1 % metal-film (or 500 Ω if verifying worst-case compliance).
- Shielded two-wire cable, ≤ 3 m for the bench test.
- STEP 7 or TIA Portal programming device with the project loaded.
- Optional: oscilloscope to verify ripple (SM332 ripple is < ±0.05 % at 4-20 mA; not necessary for a go/no-go test).
- Optional: HART communicator if downstream device is HART — confirms digital handshake over the same loop.
Do not use a current clamp on a 4-20 mA loop unless the clamp is rated for low-current DC measurement (typically 1 mA minimum). Clamps drift and saturate easily in this range.
5. Safe Loop Wiring Topology for the Test
The SM332 current-output stage sources current from the I_OUT+ terminal and returns to I_OUT- (terminals 1 / 2 of the front connector for channel 0, with offsets of 2 per channel on the 5HD01's 40-pin connector). The correct test insertion topology is series with the load:
+24 V field (not used on current loop)
|
+---[ I_OUT+ (Ch0) ]---[ 250 Ω ]---[ mA meter ]---+---[ I_OUT- (Ch0) ]---+
| |
+--------------------------- backplane GND -------------------------- +
The ammeter must be inserted in series with the load resistor, never across the output terminals. Placing the meter across the terminals short-circuits the loop and the module will current-limit at its maximum deliverable value (or display a wire-break / short diagnostic depending on configuration).
6. Step-by-Step Verification Procedure
- Power down the rack. De-energise the 24 V field supply and the S7-300 backplane power. Confirm absence of voltage with a DMM at the front connector terminals.
- Disconnect field wiring. Disconnect the channel under test from the field device (valve positioner, indicator, etc.) to isolate the SM332 output. Note terminal numbers — channel 0 uses terminals 1 (I+) and 2 (I-) of the 40-pin front connector.
- Install the test load. Connect the 250 Ω resistor across the channel's current output terminals as the loop load.
- Insert the ammeter. Break the loop and insert the DMM in mA DC mode in series with the resistor.
- Verify configuration. Confirm in HW Config / Device View that the channel is set to Current / 4-20 mA and that wire-break diagnostics are enabled if desired.
- Power up. Restore 24 V field and CPU power. Wait for the CPU to reach RUN. The SF LED of the SM332 should be OFF if the configuration downloaded correctly.
-
Force a known digital value. In the VAT / Watch table, write to the process image output word for the channel. For 4-20 mA, the digital range is 0 to 27648. Write
L#0(expect 4 mA),L#13824(expect 12 mA),L#27648(expect 20 mA). Transfer the value to the PAW usingT PAW 288(or the appropriate address). - Read the ammeter. Record the measured current for each test value. Tolerance per Siemens data sheet: ±0.5 % of full scale (20 mA), i.e. ±0.1 mA at the endpoints, plus quantisation of 1 LSB ≈ 4.8 µA.
- Sweep the range. Step the output value in 25 % increments (0, 25 %, 50 %, 75 %, 100 %) and record each reading. The curve should be linear; non-linearity > 0.5 % indicates a calibration or hardware fault.
- Disconnect and restore. Remove the test load and reconnect the field device. Document the four test readings for the maintenance record.
7. Diagnostic LED Interpretation
| SF (red) | Meaning | Action |
|---|---|---|
| OFF | Module healthy, configuration valid | None |
| Flashing | Module diagnostics pending (configuration error or wire break) | Read diagnostic buffer in STEP 7 / TIA Portal |
| ON solid | Group error: missing module, wrong parameterisation, channel fault, or internal fault | Check HW Config match, replace module if internal |
| OFF + wire-break diagnostic active | Loop open / wire break (4-20 mA only) | Check load resistor, terminal screws, cable continuity |
The 6ES7332-5HD01-0AB0 reports channel diagnostics through OB82 (diagnostic interrupt) when parameterised. Diagnostic bytes are accessible via SFC 51 / RD_REC (SFB 52) or the standard diagnostic data records (DS0/DS1). With TIA Portal, the same data is exposed in the online → diagnostics view of the device.
8. Common Wiring and Configuration Errors
| Symptom | Likely Cause | Resolution |
|---|---|---|
| Always reads 0 mA regardless of value written | Output type set to voltage; backplane not in RUN | Reconfigure channel as Current / 4-20 mA; check CPU mode |
| Reads maximum but unstable | Ammeter connected across output (short) | Re-wire ammeter in series with load |
| Reads correct value but downstream device shows zero | Field device wired to wrong terminals; loop ground missing | Verify polarity and shield/grounding per wiring diagram |
| SF LED solid, OB82 fires at startup | Configuration mismatch after firmware / project update | Recompile HW Config and re-download |
| Output stuck at ~3.6 mA regardless of PQW | CPU in STOP, output has user-defined substitute value of 0 | Switch to RUN; verify OB1 cyclic execution |
| Loop voltage exceeds 10 V at 20 mA | Load > 500 Ω; long cable run | Reduce R, use heavier gauge wire, install HART filter |
| Output noisy / > 0.5 % ripple | Inductive load, missing shield, ground loop | Use shielded twisted pair, bond shield at one end only |
9. STEP 7 / TIA Portal Configuration Verification Snippets
Quick sanity check using the VAT / Watch table in STEP 7 V5.x:
STL
L 0 // raw count = 0 → 4 mA
T PAW 288
// read mA at meter
L 13824 // raw count = 13824 → 12 mA
T PAW 288
// read mA at meter
L 27648 // raw count = 27648 → 20 mA
T PAW 288
In TIA Portal (SCL equivalent in a watch table):
SCL
"%QW288" := 0; // 4 mA
"%QW288" := 13824; // 12 mA
"%QW288" := 27648; // 20 mA
Digital-to-analog conversion formula used by the SM332 in 4-20 mA mode:
I_out (mA) = 4 + 16 × (D_in / 27648)
Where D_in is the value at the process image output word. Note that the range is 0 to 27648 (not 32767) — this is the Siemens "normalized" range for unipolar signals. Negative or values > 27648 will be clipped or reported as overshoot, depending on CPU / module firmware revision.
10. Wire-Break Detection Behavior (4-20 mA Specific)
The SM332 firmware detects wire break on current outputs by monitoring the compliance voltage. If the load drops below the threshold (typically corresponding to < 3.6 mA into a > 500 Ω path, or an actual open circuit), the module raises the wire-break diagnostic flag. With diagnostics enabled in HW Config, this triggers:
- The SF LED to flash.
- A diagnostic interrupt (OB82) with channel-specific information.
- A '1' in the corresponding channel status bit of the diagnostic data record.
This is the same diagnostic that protects the module from being driven into a saturating condition. If you need to measure the compliance voltage during testing, use the VAT to read the diagnostic buffer rather than a voltmeter across the output — the latter will load the loop.
11. Verification Checklist
- ✓ Channel configured as Current / 4-20 mA in HW Config.
- ✓ Load resistor is 250-500 Ω, rated ≥ 0.5 W, metal-film.
- ✓ Ammeter inserted in series with the load, not across terminals.
- ✓ SF LED OFF after download and CPU RUN.
- ✓ Output reads 4.0 mA ± 0.1 mA at raw value 0.
- ✓ Output reads 12.0 mA ± 0.1 mA at raw value 13824.
- ✓ Output reads 20.0 mA ± 0.1 mA at raw value 27648.
- ✓ Linearity within ±0.5 % across 25 % steps.
- ✓ Wire-break diagnostic testable: temporarily open the loop and confirm SF / OB82.
- ✓ Field wiring restored, terminal screws torqued (0.6-0.8 Nm typical for SM332 front connector).
12. Related References and Cross-Platform Notes
The general 4-20 mA verification methodology is consistent across manufacturers. For a broader overview of isolating and verifying a current-loop signal with the PLC disconnected, see the AutomationDirect application note 4-20mA Analog Transmitter Troubleshooting. For an equivalent analog output test philosophy on Allen-Bradley / Rockwell 1771 I/O, the 1771 Analog Output Module User Manual (1771-UM030) describes scaling, calibration test equipment, and load expectations in section 6. The principle of using a precision resistor as the loop load and a series ammeter (never a parallel meter) applies identically.
For users migrating an S7-300 system to an ET 200SP or S7-1500, the equivalent modules are the AQ 4×U/I ST (6ES7531-... / 6ES7532-...) on the S7-1500 ET 200SP. The output type configuration is performed the same way, but the raw range for unipolar 4-20 mA is still 0-27648. Always re-run the verification procedure when substituting modules, because the terminal assignments differ and mis-wiring is the most common cause of a zero reading after a hardware swap.
Can I measure 4-20 mA by placing a multimeter across the SM332 output terminals?
No. A multimeter in mA mode placed across the terminals short-circuits the current loop. The SM332 will current-limit and you will not see a valid process variable. Always insert the meter in series with a 250-500 Ω load resistor, never in parallel with the output.
What load resistance should I use for a verification test of the 6ES7332-5HD01-0AB0?
Use a 250 Ω, 0.5 W metal-film resistor for typical verification. The module supports up to 500 Ω maximum loop load; using 500 Ω verifies worst-case compliance at the cost of 1.0 W dissipation in the resistor. Do not exceed 500 Ω or the output will saturate.
Why is my measured current zero even though I am writing to PQW 288?
Three common causes: (1) the channel is configured as voltage instead of current in HW Config / Device View; (2) the CPU is in STOP and the substitute value is 0; (3) the field wiring is open (load disconnected). Check the SF LED first — if it is flashing, read the diagnostic buffer in STEP 7 / TIA Portal for the channel-specific error.
Is a wirewound resistor acceptable as a test load?
For a static accuracy check it is acceptable but not ideal — the inductance lengthens the loop time constant and may distort transient response. Use a metal-film or carbon-film resistor for the cleanest measurement. For static verification only, wirewound is fine.
What raw count corresponds to 4 mA, 12 mA, and 20 mA on the SM332?
The unipolar 4-20 mA range uses the Siemens normalized range 0 to 27648. Therefore 4 mA = 0, 12 mA = 13824, 20 mA = 27648. Values outside this range are clipped or reported as overshoot, depending on firmware. Use the formula I_out (mA) = 4 + 16 × (D_in / 27648).