S7-300 SM 334 4-20mA Configuration: Wiring and Scaling Method

Module Identification and Hardware Capabilities

The combination of CPU 315-2 PN/DP (6ES7 315-2EH14-0AB0) firmware V3.2 and SM 334 AI 4/AO 2 x 8 Bit (6ES7 334-0CE01-0AA0) on TIA Portal V16 Update 6 is a supported configuration, but the analog module has a constrained feature set that must be understood before searching for missing dialog options. The SM 334 8-bit variant is one of the lowest-cost S7-300 analog I/O modules and is intentionally limited in measurement type selection.

The catalog view in TIA Portal lists the module under SM 334 and provides a single entry for the 8-bit version. There is no dropdown for "Measurement type" because the module is hard-wired to interpret the analog value as 0-20 mA or 0-10 V depending on which terminals are used. The module returns a raw integer in the range 0 to 255 (8-bit), and the live-zero scaling is the programmer's responsibility.

Why the 4-20 mA Option Is Missing in TIA Portal

The user-visible absence of a 4-20 mA selection in TIA Portal's device properties is the correct behavior, not a missing dialog. The 6ES7 334-0CE01-0AA0 module does not implement the "Measuring range" parameter object that is present on SM 331 modules. Only two input modes exist:

1. Voltage mode (0-10 V): Selected by wiring the signal to the dedicated voltage input terminal pair (e.g., AI0+ terminal 1, AI0- terminal 2).
2. Current mode (0-20 mA): Selected by wiring the signal to the dedicated current input terminal pair, which engages the internal burden resistor on that channel.

The module returns a raw integer in the range 0 to 255 (8-bit). The scaling to engineering units is the programmer's responsibility, which is the only way to obtain a 4-20 mA live-zero reading on this module. This is documented in the S7-300 module data manual; for the higher-resolution SM 331 family (which provides native 4-20 mA with diagnostics), refer to the S7-300 Isolated Analog Input Module SM 331 AI 8x16 Bit reference.

Hardware Wiring for Current Mode on SM 334

Pin assignment for the 6ES7 334-0CE01-0AA0 front connector (20-pin):

For current input on AI0 (4-20 mA sensor, e.g., a pressure transmitter):

1. Connect the transmitter positive to pin 1 (AI0+).
2. Connect the transmitter negative to pin 2 (AI0-). The internal burden resistor is already wired between the V-input pin and I-input pin of the same channel; selecting the I terminals engages the burden and protects against voltage on a current-configured channel.
3. Confirm that the front connector shield bracket is bonded to cabinet ground at a single point.
4. Do not apply more than 30 mA sustained current to a current-mode input — the internal burden is sized for 20 mA nominal and will saturate above this point.

Configuring the Module in TIA Portal V16

Project setup procedure:

1. Add the CPU 315-2 PN/DP (6ES7 315-2EH14-0AB0) V3.2 from the hardware catalog. If V3.2 is not listed, install the HSP (Hardware Support Package) for the CPU from the Siemens support portal.
2. Slot 4: drag SM 334 AIO4/AO2 x 8 Bit from the catalog (order number 6ES7 334-0CE01-0AA0). Slot 4 is the default for the first SM after the CPU on S7-300.
3. Open the device view, select the SM 334, and confirm the properties dialog. Note that the "Properties > Analog inputs" tab shows only the I/O addresses (e.g., PIW 752..759) and no measurement type field. This is expected.
4. Compile the hardware configuration (HWCN) and download to the CPU.

Address mapping after compile:

Scaling 4-20 mA over a 0-20 mA Raw Input

The SM 334 returns an integer proportional to the actual current, 0-20 mA mapped linearly to 0-255. For a 4-20 mA sensor, the live-zero at 4 mA corresponds to a raw value of approximately:

Raw@4mA = (4 / 20) × 255 = 51

Raw@20mA = 255

RangeSpan = 255 - 51 = 204

Engineering units conversion (scaled to a user range, e.g., 0-100% or 0-10 bar):

Eng = ((RawIn - 51) × (EngMax - EngMin)) / 204 + EngMin

SCL / Structured Text FB (TIA Portal):

FUNCTION_BLOCK "SCALE_4_20_to_ENG"
{ S7_Optimized_Access := 'TRUE' }
VERSION : 0.1
   VAR_INPUT
      i_raw : INT;        // PIW input, 0..255
      i_eng_min : REAL;   // engineering value at 4 mA
      i_eng_max : REAL;   // engineering value at 20 mA
   END_VAR
   VAR_OUTPUT
      o_eng : REAL;       // scaled engineering value
      o_below_4mA : BOOL; // wire break / sensor loss
      o_above_20mA : BOOL;// over-range
   END_VAR
   VAR CONSTANT
      RAW_AT_4MA : INT := 51;     // (4/20)*255 rounded down
      RAW_AT_20MA : INT := 255;
      RAW_SPAN : INT := 204;      // 255 - 51
   END_VAR
BEGIN
   IF i_raw < RAW_AT_4MA THEN
      o_below_4mA := TRUE;
   ELSE
      o_below_4mA := FALSE;
   END_IF;
   IF i_raw > RAW_AT_20MA THEN
      o_above_20mA := TRUE;
   ELSE
      o_above_20mA := FALSE;
   END_IF;
   o_eng := (INT_TO_REAL(i_raw - RAW_AT_4MA) /
             INT_TO_REAL(RAW_SPAN)) *
            (i_eng_max - i_eng_min) + i_eng_min;
END_FUNCTION_BLOCK

Ladder equivalent (TIA Portal LAD) — implement the offset and span manually:

1. Read PIW into MW (e.g., MOVE PIW752 -> MW100).
2. Subtract 51 to remove the live-zero: SUB_I 51, MW100 -> MW102. Use a conditional coil to clamp to 0 on under-range.
3. Convert to REAL: DI_R MW102 -> MD104.
4. Multiply by the engineering range: MUL_R (EngMax - EngMin), MD104 -> MD108.
5. Divide by 204: DIV_R 204.0, MD108 -> MD112.
6. Add EngMin offset: ADD_R EngMin, MD112 -> MD116.

Using NORM_X and SCALE_X (TIA Portal V14+):

// Normalize 4-20 mA raw to 0.0 .. 1.0
#norm := NORM_X(MIN := 51,           // raw at 4 mA
                VALUE := "DB_AI".i_raw,
                MAX := 255);         // raw at 20 mA
// Scale 0.0 .. 1.0 to engineering units
"DB_AI".o_eng := SCALE_X(MIN := "DB_AI".i_eng_min,
                          VALUE := #norm,
                          MAX := "DB_AI".i_eng_max);

The 4-20 mA loop integrity check is critical: with this 8-bit module, raw values < 51 indicate a wire break, an open loop, or a sensor that has lost loop power. Program the under-range bit o_below_4mA into the process tag list so the HMI can flag "sensor loss" rather than reading 0%.

Alternative Siemens Modules with Native 4-20 mA Support

If 4-20 mA support is required with built-in diagnostics, wire-break detection, and direct software parameterization, replace the SM 334 8-bit module with a higher-resolution variant or an SM 331 input module.

When using the SM 331 AI 8 x 16 Bit, the manual explicitly states: "With the measuring range 4 to 20 mA and activated wire-break check, the isolated analog input module enters a wire break in the diagnostics if the process value falls below 1.185 mA (approx. 6% below 4 mA)." This is documented in the S7-300 Isolated Analog Input Module SM 331 manual (Siemens support portal).

Diagnostics, Wire-Break Detection, and Error Codes

The 6ES7 334-0CE01-0AA0 reports no module-level diagnostics. There is no SFC 51 / SZL index that surfaces a wire break or over-range status from this module. The only error visible to the user program is the raw input value itself:

If the application requires the 4-20 mA loop to be supervised for wire break, implement the alarm in user code by reading o_below_4mA from the FB block above and routing it to a process alarm OB. Because the SM 334 does not generate hardware interrupts, supervision must be done by cyclic OB1 polling (e.g., 100 ms). If a hard alarm is required, route the o_below_4mA bit to a digital output that energises a relay or an external alarm lamp.

Verification and Commissioning Procedure

1. Apply 24 VDC to the sensor loop and confirm the transmitter output (use a calibrated mA source or a milliamp meter in series).
2. Open an online watch table in TIA Portal with the PIW address (e.g., PIW 752). Force the transmitter to 4.000 mA. The raw value should read 51 ± 1 count. If it reads 0, the wiring is on the voltage terminals or the loop is open.
3. Force the transmitter to 12.000 mA. Raw value should read approximately 153.
4. Force the transmitter to 20.000 mA. Raw value should read 255.
5. Force the transmitter to 0.000 mA (open loop test). Raw value should read 0. The SCALE block output should clamp to i_eng_min and o_below_4mA should latch TRUE.
6. Capture the scaled engineering units (e.g., 0-100%) and verify linearity across at least five points: 4, 8, 12, 16, 20 mA.
7. Document the linearity table in the commissioning report. Deviation greater than ±1 LSB per step on a healthy transmitter indicates wiring noise; route the cable in a separate conduit, away from VFD power conductors.

Troubleshooting Matrix

Field-Engineering Notes

• The SM 334 AI 4/AO 2 x 8 Bit is one of the few S7-300 modules still shipped for cost-sensitive applications (HVAC, simple tank level, building automation). It is documented in the S7-300 Module Data manual under the SM 334 family.
• If 4-20 mA is mandatory and budget permits, prefer the SM 331 AI 8 x 12 Bit (6ES7 331-1KF02-0AB0) — it provides per-channel measurement type selection in HW Config and a wire-break diagnostic in 4-20 mA mode.
• For current outputs on the SM 334, the 0-20 mA range is also the only option. To drive a 4-20 mA valve, scale the output value to occupy the 51-255 raw range using the same procedure as the input block.
• The module is not channel-isolated; use a 24 VDC isolated transducer or signal conditioner if the field device is grounded at a different potential than the S7-300 cabinet.
• When replacing a faulty 6ES7 334-0CE01-0AA0 with a 6ES7 334-0KE00-0AB0 (12-bit version), recompile the HWCN and re-verify all scaling, since 12-bit raw values (0-4095) require a different scaling formula.
• For TIA Portal V16 Update 6, the GSD/GSDML import and HSP installation are unchanged from earlier V16 updates; the SM 334 8-bit module ships in the standard catalog and requires no extra HSP.
• If the application requires IEC 61131-3 structured text, the SCALE_4_20_to_ENG FB shown above can be invoked once per analog channel in OB1 (typical 100 ms cycle) or in a cyclic OB (e.g., OB35 at 100 ms).

FAQ