Reading 300°C with LOGO! via 0-10V or 4-20mA Transducer

The Siemens LOGO! 8 logic module is widely used for compact automation tasks where a full PLC is overkill, but a microcontroller is too exposed. A frequent field question is whether the LOGO! can read 300 °C sensors, given that the dedicated RTD expansion module is documented to terminate at +200 °C. The short answer is yes, with a small intermediate device. The LOGO! base and AM2 analog input module evaluate any 0-10 V or 0/4-20 mA signal regardless of what physical quantity the upstream transducer converts. The 200 °C limit only applies to the native PT100/PT1000 input on the AM2 RTD. Insert a temperature transmitter scaled to your process range, and the LOGO! reads 300 °C, 500 °C, or any span the transmitter supports.

This article covers the architecture, signal selection math, hardware wiring, LOGO! Soft Comfort scaling blocks, and field verification for a 0-300 °C measurement read by a LOGO! 8 base module through the AM2 analog expansion. Procedures apply to LOGO! 8 (6ED1052-1xx08-0BAx) with firmware 8.0 and later and to LOGO! Soft Comfort V8.x or V9.

Problem: 300 °C Exceeds the Native AM2 RTD Range

The native temperature path on a LOGO! is the AM2 RTD module (6ED1055-1MD00-0BA2 / -1MD10-0BA0). It accepts 2-wire PT100 or PT1000 sensors, and the LOGO! firmware maps the resistance value to a temperature range of -50 °C to +200 °C. Two constraints kick in at 300 °C:

• Firmware clamp: The internal scaling function in LOGO! Soft Comfort caps the displayed temperature at 200 °C. Counts beyond 200 °C return the maximum integer, so the value cannot be read out even with a sensor physically capable of the range.
• Hardware range: The PT100/PT1000 input on the AM2 RTD is rated IEC 60751 Class A or better, and the input stage can technically survive 850 °C PT100 resistance, but the LOGO! will not display it.

Adding more AM2 RTD modules does not extend the range. The cap is in the LOGO! firmware, not the module count. Therefore the only supported path to 300 °C is an external signal conditioner that linearizes the sensor into a standard analog signal the AM2 (the general-purpose analog module) understands.

LOGO! Native Temperature Capability and Hardware Limits

The relevant expansion modules for a 2-channel temperature build are listed below.

The base module (6ED1052-1xx08-0BAx) has four built-in 0-10 V analog inputs (AI1-AI4), each pulled to 0-10 VDC by a 100 kΩ impedance. For two PT100 sensors, the AM2 RTD is the obvious pick, but for 300 °C you must use the AM2 (general-purpose) module and let a transmitter do the linearization. The base module's own 0-10 V inputs can also be used if you do not need 4-20 mA or isolation.

The External Transducer Architecture

The recommended signal chain is a four-stage path with isolation and protection at every transition.

The transmitter accepts the PT100 resistance, linearizes against the IEC 60751 curve, and outputs a current proportional to temperature. Galvanic isolation at the transmitter keeps ground loops and 50 Hz pickup off the LOGO! input. Mount the transmitter in a DIN B connection head on the thermowell, or on a DIN rail inside the cabinet, depending on the part number.

Signal Selection: 0-10 V vs 0-20 mA vs 4-20 mA

The AM2 (6ED1055-1MA00-0BA0) accepts three signal modes, jumper-selected per channel. The base module's AI1-AI4 are 0-10 V only. Choose based on cable length, noise environment, and required resolution.

For 300 °C in industrial environments, 4-20 mA is the recommended default. The live-zero (4 mA offset) gives broken-wire detection: if the transmitter loses power or the cable is cut, the LOGO! reads a value below the lower range limit and triggers an Analog Threshold block. Cable lengths up to 600 m are supported with 24 VDC loop voltage. The trade-off is coarser resolution (800 vs 1000 counts), which for a 0-300 °C span is 0.375 °C/count, still adequate for most process monitoring.

Use 0-10 V only for short cabinet-internal wiring or when the transmitter has no current output option. The base module's AI1-AI4 is the cheapest route when no AM2 slot is available.

Resolution and Accuracy Calculations

The LOGO! AM2 module and the base module's analog inputs use a 10-bit ADC mapped to integer counts of 0-1000 for 0-10 V and 0-20 mA, and 200-1000 for 4-20 mA. Resolution per count is therefore span / count_span.

For a 0-300 °C transmitter, three possible readback resolutions apply.

General formula for count-to-temperature conversion in LOGO! Soft Comfort:

T(°C) = (AI_count - AI_offset) * span_degC / count_span

For 4-20 mA, 0-300 °C: T = (AI - 200) * 300 / 800 = (AI - 200) * 0.375

For 0-10 V, 0-300 °C: T = AI * 300 / 1000 = AI * 0.3

System accuracy is dominated by the PT100 sensor class and the transmitter error budget, not the LOGO! ADC. A Class A PT100 contributes ±(0.15 + 0.002·|T|) °C, so at 300 °C the probe alone is ±0.75 °C. A 0.1% FS transmitter is ±0.3 °C on a 300 °C span. Add ADC quantization and the typical end-to-end accuracy is roughly ±1.5 °C at 300 °C. This is fine for thermal limit switches and trending, marginal for closed-loop control requiring ±0.5 °C. If tighter control is required, spec a Class AA PT100 (1/3 DIN) and a 0.05% transmitter.

Selecting a 0-300 °C Transducer

For 300 °C you have two sensor options feeding the transmitter: PT100 RTD (best up to 850 °C) or Type K thermocouple (best up to 1370 °C). At 300 °C, PT100 is the better pick because the cold-junction compensation of a Type K system adds another error term. Specify the transmitter ordered with a 0-300 °C factory span or a user-configurable range.

For a 2-sensor build on a small panel, two head-mount PT100 transmitters in DIN B pockets are the cleanest wiring. The 4-wire PT100 runs into the head on the thermowell, the transmitter sits on the head, and only two wires (signal + 0 V) leave the head to the AM2. This is a standard 4-20 mA loop, powered from the AM2's +24 V analog supply terminal.

Wiring the AM2 Module to a Voltage or Current Transducer

The AM2 (6ED1055-1MA00-0BA0) terminal layout is: U+ (24 VDC out for loop), I+ (signal input), U-/I- (common 0 V). The jumper on the back of the module selects voltage (0-10 V) or current (0/4-20 mA) per channel. For a 4-20 mA loop-powered transmitter, place the jumper in the I position and wire as follows.

Steps for 4-20 mA wiring on AM2 channel 1:

1. Power down the LOGO! base and remove the AM2 module.
2. Set the channel-1 jumper on the AM2 to the I position (current input).
3. Wire the 4-wire PT100 into the transmitter head following the transmitter's data sheet (terminals 1, 2, 3, 4 for PT100).
4. Connect transmitter terminal + to AM2 terminal U+ (24 V supply).
5. Connect transmitter terminal - to AM2 terminal U- / I- (0 V). The 4-20 mA signal flows through this loop.
6. Bridge I+ to U+ on the AM2 (this is the standard 4-20 mA input configuration on AM2 where the loop is closed at the module). Verify against the AM2 wiring diagram in the LOGO! manual.
7. Re-seat the AM2, power up, and proceed to LOGO! Soft Comfort scaling.

For a 0-10 V transmitter (3- or 4-wire), the wiring differs: the transmitter needs a separate 24 VDC supply (either from the LOGO! DM16 24 V output or a 24 VDC power supply), and the AM2 jumper is set to U. The 0-10 V signal connects to I+ and the 0 V common to U-.

LOGO! Soft Comfort Scaling and Block Configuration

The AM2 analog input appears in LOGO! Soft Comfort as AI1 through AI4 for the base module and as AM2.AI1, AM2.AI2 for the expansion module. The integer count is 0-1000 for 0-10 V / 0-20 mA and 200-1000 for 4-20 mA. Convert the count to temperature with a Math block or use the Analog Amplifier / Analog Comparator / Analog Threshold blocks directly.

Scaling example for 4-20 mA, 0-300 °C:

1. Open the program in LOGO! Soft Comfort V8.4 (or current version).
2. Drag a Math block onto the diagram. Set Gain = 0.375 and Offset = -75. This maps 200-1000 to 0-300 °C.
3. Connect AM2.AI1 to the Math input.
4. Drag a Display element (or Message Text on the LOGO! TD) and bind the Math output. Format as AI1 = ###.# °C.
5. For alarm, drag an Analog Threshold block. Set On threshold = 280, Off threshold = 270. Tie the output to a digital output or message.
6. For broken-wire detection, use a second Analog Threshold with On = -5, Off = 1. If the count drops below 200 (e.g. cable cut), the scaled value goes negative and trips the alarm.

// LOGO! Soft Comfort Math block parameters
Gain (m):    0.375
Offset (b): -75.0

// Mapping verification
// AI = 200  -> (200 * 0.375) - 75 = 0   °C
// AI = 600  -> (600 * 0.375) - 75 = 150 °C
// AI = 1000 -> (1000 * 0.375) - 75 = 300 °C

Scaling example for 0-10 V, 0-300 °C:

// LOGO! Soft Comfort Math block parameters
Gain (m):  0.30
Offset (b): 0.0

// Mapping verification
// AI = 0    -> 0   °C
// AI = 500  -> 150 °C
// AI = 1000 -> 300 °C

Save the program to the LOGO! storage card or transfer via Ethernet. LOGO! 8 base modules with Ethernet support remote program push from Soft Comfort over the LAN, which is the recommended method for hot-standby spares.

Commissioning, Calibration, and Verification

A 2-channel 0-300 °C build with head-mount transmitters and AM2 should be commissioned in five steps.

1. Sensor simulation: Disconnect the PT100 and apply a precision decade resistance box (or a 0-300 °C dry-block calibrator) at the transmitter input. Verify the transmitter current is 4 mA at 0 °C and 20 mA at 300 °C, with linearity better than ±0.1% of span. Most head-mount transmitters include a built-in loop test feature.
2. Wiring verification: With the LOGO! powered, measure DC voltage at the AM2 input. For 4-20 mA mode, drop across the sense resistor is typically 0.5-4.5 VDC corresponding to 4-20 mA. For 0-10 V mode, the input voltage should match the transmitter output to within ±10 mV.
3. LOGO! scaling check: In LOGO! Soft Comfort online mode (or via the LOGO! TD on-line menu), read the AI value at 0 °C, 150 °C, and 300 °C. Confirm the scaled value is within ±0.5 °C of the simulation setpoint. The 0.5 °C check is tighter than 0.375 °C resolution because you are including ADC and scaling rounding.
4. Alarm path test: Force the AI count to 199 (just below the live zero). Confirm the broken-wire alarm fires. Force the AI count to 933 (300 °C). Confirm the over-temperature alarm fires.
5. End-to-end test: Install the PT100 in a heated bath at 100 °C, 200 °C, and 300 °C. Allow 5 minutes of stabilization at each point. The LOGO! displayed value should be within ±1.5 °C of the bath reference (typical Class A PT100 + 0.1% transmitter + 10-bit ADC stack).

Document the as-left values, calibration date, and probe serial numbers in the panel dossier. Most plant QA systems require a calibration certificate traceable to NIST or PTB for process-critical loops, even for a 2-channel monitor.

Troubleshooting Matrix

Common faults and the diagnostic path for a 0-300 °C LOGO! build are listed below.

Frequently Asked Questions