Getting the speed reading into your control system

A tachometer can provide analog current or voltage, speed pulses, relay or transistor alarms, and digital communications, depending on its configuration. Some instruments offer a local readout only. Check the complete order code for fitted outputs and permitted combinations. A feature listed for a product family may require a separate option.

Match the output to the information you need

Scaling an analog signal into rpm

An analog output maps speed onto a current or voltage range. Consider an illustrative setting of 0–3000 rpm across 4–20 mA: 4 mA represents zero speed, 12 mA represents 1500 rpm, and 20 mA represents 3000 rpm.

n = (I − 4) / 16 × 3000
I is current in mA; n is speed in rpm. Configure the PLC analog input with matching endpoints. This example is a chosen scaling, not a universal factory setting.

Zero speed can legitimately produce 4 mA. A reading near 0 mA may indicate a broken connection, lost power or another problem. Interpret it using the instrument's specified fault behaviour; defined fault currents and automatic wire-break diagnostics are not available on every tachometer.

Current and voltage interfaces require compatible circuits. Passing 4–20 mA through a suitable 250 Ω resistance produces 1–5 V, not 0–10 V. Check resistance tolerance, input loading, total loop resistance and the output's drive capability. Follow the manual for active/passive connections, power requirements and signal common.

A pulse train is different from a switched alarm

For a pulse output, establish whether it repeats detection pulses or generates a scaled frequency. Where the pulses directly represent shaft rotation, n = 60 × f / P, with f in Hz and P in pulses per revolution. For example, 50 Hz at two pulses per revolution means 1500 rpm. An incorrect encoder pulse count or reflective-marker count changes the calculated result.

NPN/PNP identifies the electrical arrangement, not the output's purpose. An NPN or PNP output can simply switch when a speed limit is crossed. Check sinking/sourcing compatibility, voltage levels, pulse width and the receiver's maximum frequency.

As a documented example, the HIOKI FT3406 provides a selectable 0–1 V analog output or 0–3.3 V detection-pulse output through one shared port. They cannot operate simultaneously. The pulse is active low, with a 300 µs low interval. Do not assume compatibility with a 24 V PLC input: check its thresholds and whether a signal converter is required. These outputs are not provided on the FT3405.

Comparison outputs report a condition

Relay and transistor comparison outputs can report low speed, overspeed or an acceptable range for the application. A single on/off output does not convey the actual rpm value. Where supported, hysteresis and delays help prevent repeated switching near a threshold. An illustrative high-speed alarm might switch on at 1800 rpm and release at 1750 rpm; verify the available comparison logic.

During installation, check contact ratings for the actual load type, and transistor voltage, current and leakage limits. A relay contact switches an external circuit; it does not supply power itself. A transistor output also needs a compatible load and common connection. Measurement accuracy, output response time and filtering affect the resulting alarm behaviour.

Communications and option cards need separate checks

A supported communications interface can transfer speed and status to a PLC, HMI or SCADA system. RS-485 specifies an electrical interface; Modbus RTU specifies a protocol. An RS-485 connection alone does not prove Modbus support. Confirm data addresses, units, scaling and the action taken when communication stops.

The OMRON K3HB-R family offers analog current/voltage, relay, NPN/PNP comparison, BCD and serial communication options. However, linear analog output and RS-232C/RS-485 cannot be used together in one instrument; check the order-code combinations. For Red Lion PAXI instruments, the option card matters too: the manufacturer specifies PAXCDC10 for Modbus operation, while PAXUSB00 is a programming interface. A USB connection should not be assumed to provide a continuous measurement stream.

Specify the receiving equipment before ordering

Define the receiver's input type, signal range, rpm scaling, number of outputs and simultaneous-use requirements. Check the measurement range separately from the range assigned to the analog output. Assess galvanic isolation and operating conditions. At commissioning, use a known speed or an appropriate output-test function to compare the local display with the receiving system. During maintenance, a working display alone does not establish that the output remains accurate.

Related connection and measurement guides

Sensor inputs · Communication options · Installation and wiring · Troubleshooting · Response time

Analog signals · Analog inputs · Sinking and sourcing · Tachometer questions and answers

Technical references: OMRON K3HB-R: output options and combinations · HIOKI FT3405/FT3406: analog and pulse outputs · HMS / Red Lion: PAX option cards