Siemens / SIMATIC S7-300 / SM 331

Every SM 331 variant including RTD and thermocouple, plus the HART modules. Thirteen article numbers on one page.

The SM 331 is the analog input module family for the SIMATIC S7-300 rack. It converts continuous process signals into values the CPU can use: voltage, current, resistance, resistance thermometers and thermocouples. Variants differ by channel count, resolution, measurement type and speed.

This is the family with the most variants and the most ways to get a substitution wrong. Resolution and measurement type both matter, and two modules with the same channel count can be completely different parts.

SM 331 analog input modules

Taken from the Siemens S7-300 Module Data equipment manual.

Process automation variants

These are documented in the separate Siemens ET 200M Signal Modules for Process Automation manual. New Zealand process plants run a lot of HART instrumentation and very few suppliers list these.

The two that catch people out

6ES7331-7PF01-0AB0 is RTD. 6ES7331-7PF11-0AB0 is thermocouple. One digit apart, both 8-channel temperature modules, completely different sensor types. An RTD module will not read a thermocouple and vice versa.

6ES7331-7NF00-0AB0 and 6ES7331-7NF10-0AB0 are both AI 8 x 16 bit. The 7NF10 is the later generation with different parameter handling. Check which one the hardware configuration expects.

Send us both numbers and we will tell you whether the swap will work before you buy.

SIPLUS equivalents

Extended environmental range, minus 25 to plus 60 or 70 degrees C, condensation permitted. Same function, different article number.

Why your reading is drifting

Most analog faults are not the module. Before you replace one, check these.

Isolated versus non-isolated transducers. Siemens documents the two wiring arrangements separately for a reason. Wiring an isolated transducer as a non-isolated one, or the reverse, produces readings that drift and float rather than fail outright. This is the most common analog problem there is.

Cold junction compensation on thermocouples. Thermocouple modules support internal or external compensation. Get it wrong and you see a consistent offset, sometimes tens of degrees, that people chase for days assuming a faulty sensor.

Resistance thermometer wiring. Two, three and four conductor connections are all supported and give different accuracy. A two-wire connection includes the cable resistance in the measurement.

Conversion and cycle time. Analog modules are not instant. If a control loop is unstable, the module scan time may be the reason rather than the tuning.

Front connectors

Every SM 331 ships without one.

ET 200M uses the same modules

An ET 200M station is an IM 153 interface module plus ordinary S7-300 signal modules. The ET 200M also acts as HART master for HART field devices in hazardous areas, with the IM 153-2 passing parameter data through the HART analog input module. See S7-300 parts.

Send us your article numbers

One module or a full project list. We will come back with price, condition and lead time. Quotes in NZD including GST.

Request a quote

Industech is an independent supplier of new, surplus and used industrial automation equipment. We are not an authorised distributor of, approved by, sanctioned by, or affiliated with Siemens AG. All trademarks, brand names and article numbers are the property of their respective owners and are used here for identification purposes only.