Overview: SB 1232-AQ Signal Board on the S7-1200
The SB 1232-AQ is a single-channel, 12-bit analog output Signal Board (SB) that plugs into the top of any S7-1200 CPU (firmware V4.0 and later for most variants). The "SB" prefix is Siemens' designation for plug-in expansion boards that do not require a separate slot or backplane — they sit on top of the CPU body. Because the SB 1232-AQ is single-channel, it does not follow the two-LED-per-channel pattern of the larger SM 1232 modules; the channel-status indication is consolidated into a single multi-color STATUS LED on the board itself, while the module-level DIAG LED is hosted on the CPU.
Key identity data:
- MLFB / order number: 6ES7232-4HA30-0XB0
- Firmware: depends on the CPU that hosts it (no independent SB firmware)
- Output count: 1
- Output type: voltage OR current (selectable per channel, not simultaneous)
- Output range: ±10 V or 0 to 20 mA (4–20 mA is achieved by scaling the lower 25% of the 0–20 mA span)
- Resolution: 12 bits in voltage mode, 11 bits in current mode
- Update time: 1 ms (typical) on a 100 ms analog base
- Load impedance: voltage ≥ 1 kΩ, current ≤ 600 Ω
Source: TIA Portal Manual Collection – SB 1232 1 AO Specifications.
LED Inventory on the SB 1232-AQ
The board carries one user-visible STATUS LED for the analog output channel. Despite the field question implying "8 LEDs", a physical count of an installed SB 1232-AQ will reveal one LED; the apparent LED count in photographs of populated CPU+SB assemblies comes from the CPU's own DIAG/MAINT/RUN/ERROR LEDs and the SB 1232-AQ's STATUS LED combined. Engineers should map indicators as follows:
| LED | Location | Function |
|---|---|---|
| STATUS (green/red) | SB 1232-AQ board | Channel-level status: ON (green) = output active and valid; OFF = output not driven or module not configured; FLASHING red = diagnostic event |
| DIAG (red/green) | CPU top cover | Module-level fault indicator, not on the SB itself |
| RUN/STOP, ERROR, MAINT | CPU | CPU state, not SB-specific |
The single STATUS LED supports two colors and three steady/flash states per color, which is what enables it to encode both normal operation and the two diagnostic conditions available on analog outputs.
Available Diagnostics on Analog Outputs vs. Analog Inputs
A common point of confusion is that the diagnostics properties dialog in TIA Portal exposes overflow and underflow options for analog inputs only. On the SB 1232-AQ (an analog output), the supported diagnostics are mode-dependent:
| Output Mode | Available Diagnostic | Trigger Condition |
|---|---|---|
| Voltage (±10 V) | Short-circuit detection | Output current exceeds the hardware short-circuit threshold (typically > 25 mA on the voltage output driver) |
| Current (0/4–20 mA) | Broken-wire detection | Loop impedance exceeds the open-loop detection threshold while the output is being driven above 0 mA |
Overflow and underflow diagnostics listed under the signal board's properties in TIA Portal are greyed-out or absent because they apply only to the input range. Confirming this behavior is a routine part of commissioning — if a trainee engineer sees overflow/underflow options enabled in the dialog, the wrong module type has been inserted in the device configuration.
Configuring Diagnostics in TIA Portal
To reach the diagnostic options for the SB 1232-AQ:
- Open the project in TIA Portal and expand the device view of the S7-1200 CPU.
- Click the SB 1232-AQ slot on top of the CPU. The inspector on the right shows Properties > General > Analog outputs.
- Open Diagnostics. The active checkbox depends on the output mode selected under Output type:
- Voltage selected → Short-circuit diagnostics is the only enabled checkbox.
- Current selected → Broken-wire diagnostics is the only enabled checkbox.
- Compile and download the hardware configuration. The diagnostic becomes effective on transition of the CPU from STOP to RUN.
STATUS LED Behavior and Diagnostic Mapping
The STATUS LED on the SB 1232-AQ behaves as a tri-state indicator per the S7-1200 LED system. The exact mapping the firmware drives is:
| STATUS LED | Meaning | Likely Cause |
|---|---|---|
| OFF | Output not active, no value being written | CPU in STOP, channel disabled, or output value set to 0 in user program while output is configured but idle |
| Solid green | Output active, no diagnostic pending | Normal operation; the scaled value being written to the process image is within range |
| Flashing red | Diagnostic event active (short-circuit in voltage mode, broken wire in current mode) | Wiring fault, load out of range, or field loop interrupted |
| Solid red (rare on SB) | Module fault | Hardware defect, firmware/parameterization mismatch |
When the STATUS LED is flashing red, the corresponding diagnostic bit in the process image is set. Read the diagnostic interrupt OB (OB82) to retrieve the channel status word and identify whether the fault is short-circuit (voltage) or broken-wire (current). Refer to the TIA Portal help text on the signal board for the exact bit mapping.
Wiring the SB 1232-AQ
The SB uses a removable, spring-loaded or screw-type 3-terminal block (depending on the variant ordered). The terminal assignments are:
| Terminal | Signal | Notes |
|---|---|---|
| 1 | AQ — analog output (V or I) | Shielded twisted pair recommended for long runs |
| 2 | MANA — analog ground | Tied to CPU M internally; do not bond to protective earth at the field end |
| 3 | — / shield | Shield termination point; ground at one end only |
Wiring diagram and identification/maintenance (I&M) data access is documented in the S7-1200 manual collection: SB 1232 1 AO Wiring Diagram (TIA Portal Manual Collection).
Voltage Output Connection
Connect terminal 1 to the positive input of the field device, terminal 2 to the negative input. The load must present ≥ 1 kΩ to the output to avoid engaging the short-circuit detector. For 4-wire field devices, jumper the device's signal-return to MANA locally at the field end.
Current Output Connection
Connect terminal 1 to the positive input of the field device, terminal 2 to the negative input. The load (including cable resistance) must be ≤ 600 Ω. The broken-wire detector works by sensing that the commanded current cannot be developed; a loop with an open conductor or a load > 600 Ω triggers the diagnostic.
Scaling and Process Image
The SB 1232-AQ occupies two words in the output process image of the CPU. The nominal integer range depends on the mode:
| Mode | Engineering Range | Integer Range | Scaling Formula (value → engineering units) |
|---|---|---|---|
| Voltage ±10 V | −10 V … +10 V | −27648 … +27648 | V = (value / 27648) × 10 |
| Current 0–20 mA | 0 mA … 20 mA | 0 … 27648 | mA = (value / 27648) × 20 |
| Current 4–20 mA (scaled) | 4 mA … 20 mA | 0 … 27648 | mA = 4 + (value / 27648) × 16 |
The 12-bit voltage resolution yields 4096 discrete steps (LSB ≈ 4.88 mV); the 11-bit current resolution yields 2048 steps (LSB ≈ 9.77 µA). The unused LSBs in current mode are zero-padded in the process image.
Troubleshooting Matrix
| Symptom | STATUS LED | CPU DIAG LED | Probable Cause | Corrective Action |
|---|---|---|---|---|
| Output stuck at 0 V / 0 mA | OFF | OFF | CPU in STOP, or no value being written to %QW | Switch CPU to RUN; verify the program writes the output tag every cycle |
| Output at full-scale regardless of value | Solid green | OFF | Scaled value clipped to 27648, or wrong scaling type chosen in TIA Portal | Re-check the scaling block and the signal board's output type |
| Output unstable, drifting | Solid green | OFF | EMI on analog cable, missing shield termination | Use shielded cable, ground shield at one end only, separate from power cables |
| Output reads correct value with DMM, but field device reads 0 | Solid green | OFF | Wiring reversal or break between SB and field device | Check terminal 1/2 polarity and continuity end-to-end |
| STATUS LED flashing red, voltage mode | Flashing red | Red (if OB82 not loaded) | Short-circuit on the analog output | Disconnect the field wiring and measure loop resistance; if it returns to normal, the field cable or device is shorted |
| STATUS LED flashing red, current mode | Flashing red | Red (if OB82 not loaded) | Broken wire, open loop, or load > 600 Ω | Inspect terminations; measure loop resistance end-to-end; check that the load is within spec |
| STATUS LED solid red | Solid red | Red | Module internal fault, parameterization mismatch | Power-cycle the CPU; re-download the hardware configuration; replace SB if persistent |
| OB82 fires cyclically with no LED change | Solid green | Off or red | Channel diagnostic interrupt enabled in OB82 but the field is on the edge of a limit | Add hysteresis in the user program; check that the loop is not oscillating |
Reading Diagnostics in the User Program
Diagnostic events raise OB82. Inside OB82, evaluate LADDR (the hardware identifier of the SB) and read the IO2.PIP4 diagnostic status word. The bits you can decode:
- Bit 0 — module fault
- Bit 1 — internal error
- Bit 2 — external error (e.g., short-circuit / broken-wire)
- Bit 3 — channel fault
Pair OB82 with a status word read via RDREC (SFB52) on the diagnostic data record 0. A typical SCL snippet for latching the diagnostic state:
// OB82 — diagnostic interrupt
IF #LADDR = "SB1232_AQ_HS" THEN
"DB_Diag".SB_ExtFault := TRUE;
"DB_Diag".SB_ChannelFault := TRUE;
END_IF;Reset the latches on the rising edge of the next successful read of the output process image, or in the user program when the command value is back in range and the loop is verified.
Commissioning Procedure
- Insert the SB 1232-AQ in the device view of TIA Portal. Confirm MLFB 6ES7232-4HA30-0XB0.
- Select output type (Voltage or Current) and the appropriate engineering range. Note: changing output type from the portal requires a STOP→RUN transition.
- Enable the appropriate diagnostic (Short-circuit or Broken-wire) under Properties > Diagnostics.
- Wire the SB following the terminal map above. Use shielded twisted pair and ground the shield at the cabinet end only.
- Compile the hardware configuration, download to the CPU, and switch to RUN.
- Force a known value to the output word and measure with a calibrated DMM. Confirm the engineering units match the formula in the scaling table.
- Verify the STATUS LED is solid green for normal operation.
- Verify the diagnostic trip path: in voltage mode, short terminal 1 to terminal 2 with a jumper — STATUS should flash red and OB82 should fire. In current mode, open the loop at terminal 1 — STATUS should flash red and OB82 should fire.
- Remove the test condition, confirm the LED returns to solid green, and record the calibration in the plant's loop folder.
Field-Proven Caveats
- Mode switch requires a STOP transition. TIA Portal will refuse to apply a new output type online; you must place the CPU in STOP, apply, and return to RUN.
- Broken-wire detection is only meaningful above 0 mA. Driving 0 mA with the loop open is the normal current-loop idle state and will not flag as a fault.
- Short-circuit detection in voltage mode can trigger on highly capacitive loads. A long cable to a slow field device can present a momentary inrush that trips the detector at start-up. Add a series resistor at the output only if the device's input impedance is otherwise too low.
- Sharing MANA with digital outputs creates ground loops. If the SB is wired near a digital output, run a dedicated analog return conductor and bond MANA at one point.
- SB is keyed to the CPU model. Older CPU firmware (V1.0/V2.0) does not recognize the SB 1232-AQ. Confirm the CPU firmware is V4.0+ before installing.
Related Documentation
- S7-1200 Manual Collection — SB 1232 1 AO Technical Specifications
- S7-1200 Manual Collection — SB 1232 1 AO Wiring Diagram and I&M Data
- Siemens Industry Online Support (search "6ES7232-4HA30-0XB0" for firmware, downloads, and FAQs)
FAQ
How many LEDs does the SB 1232-AQ have and what do they indicate?
The SB 1232-AQ has one user-visible STATUS LED on the board itself. Solid green = output active and valid, OFF = output not driven, flashing red = active diagnostic (short-circuit in voltage mode, broken wire in current mode). The DIAG LED is on the CPU, not on the SB.
Why are overflow and underflow diagnostics greyed out on the SB 1232-AQ?
Overflow and underflow diagnostics apply only to analog input modules. On the SB 1232-AQ analog output, the only available diagnostics are short-circuit (voltage mode) and broken-wire (current mode), selected under the signal board's properties in TIA Portal.
What is the resolution of the SB 1232-AQ?
12 bits in voltage mode (LSB ≈ 4.88 mV on the ±10 V range) and 11 bits in current mode (LSB ≈ 9.77 µA on the 0–20 mA range). The full-scale integer range is −27648 to +27648 in voltage mode and 0 to 27648 in current mode.
Can the SB 1232-AQ drive 4–20 mA?
Yes. The hardware range is 0 to 20 mA; a 4–20 mA signal is produced by scaling the integer value so that 0 corresponds to 4 mA and 27648 corresponds to 20 mA: mA = 4 + (value / 27648) × 16.
Which CPU firmware versions support the SB 1232-AQ?
Generally CPU firmware V4.0 and later. Older firmware (V1.0/V2.0) does not recognize the SB. Always confirm the CPU's firmware level in the device view of TIA Portal before installing the SB 1232-AQ.