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| Place of Origin | Germany |
| Brand Name | SIMENS |
| Certification | CE RoHS |
| Model Number | 6ES7350-2AH01-0AE0 |
The Siemens 6ES7350-2AH01-0AE0 is the FM 350-2 — Siemens's eight-channel counter and measurement function module for the SIMATIC S7-300 system.
Where the built-in high-speed counter inputs of compact S7-300 CPUs offer two channels at 10kHz, the FM 350-2 raises the capability to eight independent channels at 20kHz, each capable of operating in its own independently selected mode, independently configuring its own gate functions and output assignments.
For machines that need to monitor multiple encoders, count products at multiple measurement points, or measure speed on several machine axes simultaneously, the FM 350-2 replaces what would otherwise require a collection of separate intelligent counting hardware.
The eight channels operate entirely in the FM 350-2's own processor — independent of the S7-300 CPU's scan cycle. An encoder signal at 20kHz completes a full pulse in 50 microseconds; no PLC scan cycle running at 10ms could reliably capture every pulse at this rate using normal digital input processing.
The FM 350-2's hardware counters latch every pulse regardless of the CPU's current workload, and the counted value, measured frequency, or computed speed is available in the module's buffer memory for the CPU to read on the next scan — accurate to the last pulse.
The five operating modes — counting, frequency measurement, speed measurement, period duration measurement, and dosing — make the FM 350-2 a multi-purpose measurement engine within the S7-300 rack.
Each channel's mode is set independently, so a single FM 350-2 can simultaneously run one channel as a length counter for a conveyor belt, another as a frequency meter for a flow sensor, and two more as speed monitors for different machine axes — all from the same module.
| Parameter | Value |
|---|---|
| Channels | 8 |
| Max. Frequency | 20kHz |
| Encoder Types | NAMUR, 24VDC |
| Digital Inputs | 8 × NAMUR, 20kHz max |
| Digital Outputs | 8 × 24VDC, 500mA |
| Counter Width | 32-bit |
| Operating Modes | Count, freq., speed, period, dosing |
| Module Width | 80mm |
| Weight | 0.5kg |
| Connector | 40-pin (ordered separately) |
Counting is the most fundamental mode. The counter accumulates pulses from the encoder input, incrementing or decrementing the 32-bit counter value on each pulse edge. Gate inputs (external signals or software commands) control when counting is active.
At the end of a counting interval or when a preset value is reached, the configured output activates — triggering a gate, a relay, or an alarm in the machine. Continuous, single, and periodic counting variants cover different application needs.
Frequency Measurement counts pulses over a defined gate time and reports the frequency in Hertz. The gate time is configurable — longer gate times give better accuracy at low frequencies, shorter gate times give faster response at high frequencies.
A flow sensor generating 100 pulses per litre at a flow rate of 5 litres per minute produces 500 pulses per minute (8.33Hz); the FM 350-2 measures this and provides the frequency value for flow rate calculation in the CPU program.
Speed Measurement extends frequency measurement by converting the pulse frequency to a speed value using the configured encoder resolution (pulses per revolution). The CPU reads the speed register in the FM 350-2's data buffer and has a direct RPM or peripheral speed value for display, logging, or closed-loop control.
Period Duration Measurement measures the time between successive encoder pulses, providing an inverse-frequency measurement that is more accurate at low speeds than direct frequency counting.
At very low speeds — where a counting gate of one second might capture only a few pulses and thus give poor frequency resolution — measuring the period between pulses gives much higher accuracy.
Dosing Mode is the FM 350-2's most application-specific feature. The module counts pulses from a flow meter, conveys the accumulated total, and activates an output when a programmed total count (the "dose") is reached.
The output directly controls the dispensing valve, solenoid, or pump — without routing through the PLC scan cycle.
This hardware-direct dosing avoids the latency of reading the count in the PLC program and writing the output in the next scan, which is critical in high-speed dosing applications where a few milliseconds of latency represents significant volume error.
The FM 350-2's input interface accepts NAMUR sensors — a two-wire interface defined in IEC 60947-5-6, widely used for proximity sensors in hazardous area installations.
NAMUR sensors are powered by the field device (not the sensor itself) and vary their current consumption to represent the two logical states: approximately 1mA for the inactive state and approximately 8mA for the active state.
The NAMUR interface on the FM 350-2 evaluates this current variation, allowing connection of sensors installed in Zone 1 or Zone 2 hazardous areas when used with appropriate NAMUR-compatible intrinsic safety barriers (Zener barriers or galvanic isolators) between the sensor and the module.
This makes the FM 350-2 suitable for speed measurement and counting in chemical process plants and refineries where rotating machinery in hazardous areas requires speed monitoring.
Standard 24V DC proximity switches and encoders can also connect to the FM 350-2's inputs, operating in the 24V encoder mode that accepts standard PNP-type digital signals rather than NAMUR current signals.
The FM 350-2's eight digital outputs operate at up to 500Hz switching frequency and can activate based on the counter reaching a programmable comparison value, without CPU intervention. This hardware interrupt output capability is the key to dosing and comparison-triggered operations where cycle time matters.
In a comparison configuration, the CPU writes a target count value to the FM 350-2's comparison register via K-Bus communication.
As the counter accumulates pulses, the FM 350-2 internally compares the running total against the target. When they match, the FM 350-2 directly activates the corresponding output — no PLC scan cycle involvement.
The output reaction time is hardware-determined: typically in the microsecond range for the comparison logic and the output driver, rather than the millisecond-range PLC scan cycle time that would apply if the CPU had to read the count, compare it in the program, and write the output in a subsequent scan.
Q1: Does the FM 350-2 require any specific front connector or encoder cable, or are these standard S7-300 accessories?
The FM 350-2 uses the standard 40-pole S7-300 front connector (6ES7392-1AM00-0AA0 for screw contacts, or 6ES7392-1BM01-0AA0 for spring contacts) — the same connector used by other 40-pole S7-300 signal modules. It is ordered separately and is not included with the module.
For encoder wiring, standard shielded twisted-pair cable is appropriate; no proprietary cable or specialised encoder connector is required.
The wiring connects to the 40-pole front connector terminals according to the FM 350-2's terminal assignment table in the configuration documentation.
Q2: Can the FM 350-2 be installed in an S7-300 expansion rack connected via IM 361, or must it be in the central rack?
The FM 350-2 is a function module (FM) that communicates with the CPU via the K-Bus. The K-Bus is routed to expansion racks connected via the IM 360/361 pair, but not via the simpler IM 365 pair.
Therefore: the FM 350-2 can be installed in expansion racks connected with IM 361 interface modules (K-Bus present), but cannot operate correctly in expansion racks connected via IM 365 (no K-Bus).
This is one of the application-specific constraints that determine whether the IM 365 or IM 360/361 pair is appropriate for a given S7-300 multi-rack configuration.
Q3: What configuration software is included with the FM 350-2, and is it compatible with TIA Portal?
The 6ES7350-2AH01-0AE0 includes a configuration package on CD-ROM — this is the STEP 7 function block library and configuration tool specific to the FM 350-2, used with STEP 7 Classic (v5.x).
TIA Portal support for the FM 350-2 is available through STEP 7 in TIA Portal (when using TIA Portal together with the STEP 7 Professional package) or through the FM 350-2's TIA Portal configuration tool available from Siemens Industry Online Support.
For systems already engineering in STEP 7 Classic, the included CD-ROM provides all necessary configuration tools; for TIA Portal users, the appropriate TIA Portal-compatible package should be verified and sourced from Siemens.
Q4: Can the FM 350-2's channels be used simultaneously in different operating modes?
Yes. Each of the eight channels is independently configurable. Within a single FM 350-2 module, channels can simultaneously operate in different modes — for example, channels 1–4 in continuous counting mode for production quantity monitoring, channels 5–6 in speed measurement mode for monitoring motor speeds, and channels 7–8 in dosing mode for dispensing control.
The configuration is set during commissioning via the configuration tool and downloaded to the module.
The CPU program reads data from each channel independently, accessing the appropriate data block structures for each channel's operating mode.
Q5: What happens to the FM 350-2's counter values if the S7-300 CPU goes to STOP mode?
When the S7-300 CPU transitions to STOP, the FM 350-2's counter hardware continues operating — the counters keep running, accumulating pulses from connected encoders.
The CPU's communication with the FM 350-2 via the K-Bus ceases in STOP mode, so the CPU cannot read updated counter values or write new setpoints during the STOP period.
However, the FM 350-2 itself and its hardware counters remain powered and active. When the CPU returns to RUN, K-Bus communication resumes, and the CPU can read the current counter values, which reflect the counts accumulated during the STOP interval.
This behaviour is important for dosing and counting applications where material may continue moving through the machine during a brief CPU STOP — the count data is not lost.
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