Get the output circuit wrong and one of two things happens. A driven-voltage output on an input built for a contact can damage the input. A transistor output with no supply, or with its polarity reversed, gives no counts. The meter's labels do not prevent either error, because "passive", "volt-free" and "S0" describe different circuits in different manuals. Identify the circuit from the terminal diagram first. Then check timing and pulse weight as separate steps.
Three output circuits
| Output circuit | What the meter does | What the receiving circuit needs |
|---|---|---|
| Mechanical dry contact, such as a reed switch or relay | Opens and closes a contact without supplying the signal voltage | A sensing supply, suitable contact load and bounce handling |
| Open-collector or open-drain transistor | Switches current through an externally supplied circuit | The specified supply and current limiting, correct polarity and compatible thresholds |
| Driven-voltage output, such as push-pull | Drives defined high and low levels from its output supply | An input rated for those levels and the specified signal reference |
IEC 62053-31 calls the S0 output "passive" because the meter does not power it. Some manuals use "passive" only for a relay contact and call any transistor output "active". Read the circuit, not the word.
A solid-state output sold as "volt-free" still has polarity, off-state leakage and an on-state voltage drop. A relay contact has none of these. Wire a solid-state "volt-free" output as a transistor output.
Dry contacts
A reed switch or relay contact supplies no voltage. Closing it completes the receiver's sensing circuit. A bare contact pair has no polarity, but protection components on the meter's terminals can add one. Follow the complete terminal diagram.
The contact must switch the receiver's small sensing current reliably. Power-relay contacts are rated in amps, and at a few milliamps a surface film on the contact can stop it closing the circuit. Omron's G2R datasheet shows the size of the difference. The 10 A single-contact version has a reference minimum load (failure rate P level) of 100 mA at 5 V DC. The bifurcated-contact version has 1 mA at 5 V DC. An input that wets the contact with a few milliamps needs a reed switch or a bifurcated or gold-clad contact, not a power contact.
Contacts bounce: one closure produces a short burst of transitions. The input filter must reject the burst and still accept the shortest valid pulse. OMRON's relay guidance treats contact load and bounce as separate selection checks. Bounce that gets through the filter shows as counts that the meter's own register does not have.
Open-collector and open-drain outputs
An NPN open-collector or N-channel open-drain output pulls the signal towards its return when it switches on. When it switches off, it releases the signal, and a pull-up in the receiver or an external resistor sets the other state. Many counters already have that pull-up inside. Read the input specification before you add a supply or resistor.
Open collector is the bipolar transistor form; open drain is the MOSFET form. A PNP or P-channel output sources current instead of sinking it. Identify the transistor type and the current direction from the terminal diagram.
The off-state leakage and the on-state voltage drop must leave margin on each side of the receiver's threshold. Texas Instruments' application note SLVA485 shows how the pull-up value, the input thresholds, leakage and line capacitance set that margin and the rise time. The S0 section below works one example with numbers.
Many transistor outputs are optically isolated from the meter's internal circuits. Schneider Electric documents the iEM2050 pulse output as optically isolated, polarity-dependent and open collector, with an external voltage source required. An output like this does not share the meter's internal ground. Connect its negative terminal to the receiver's return, not to another terminal labelled common.
Driven-voltage outputs
A push-pull or other driven output sets its own high and low levels. Check the high level under the receiver's load, the receiver's maximum input voltage and the return connection. Do not connect a driven output to an input designed only for contacts unless the input specification permits it. An input that sources its own sensing current can have current forced back into it by a driven high level.
"Active energy pulse output" usually means pulses that represent active energy in kWh. The phrase says nothing about the electrical driver. Read the electrical specification as well as the measured quantity.
S0 outputs
S0 is a two-wire, polarity-sensitive pulse output powered by an external supply. The switch is usually an optocoupler transistor. The name comes from DIN 43864. IEC 62053-31:1998 defined the interface internationally, and IEC withdrew it in 2022 when IEC 62052-11:2020 replaced it. Meter manuals still cite 62053-31.
The interface has two classes. Class A is for long cable runs and class B for short ones. The values below are from the Saia Burgess Controls S0 application note, which summarises the standard.
| Parameter | Class A | Class B |
|---|---|---|
| Maximum supply voltage | 27 V DC | 15 V DC |
| On-state current | 10 mA to 27 mA | 2 mA to 15 mA |
| Maximum off-state current | 2 mA | 0.15 mA |
| Minimum pulse and gap | 30 ms each | 30 ms each |
A 30 ms pulse plus a 30 ms gap gives a 60 ms period, so an S0 output cannot exceed about 16.7 Hz. At 1,000 pulses/kWh the output reaches that rate at 60 kW. For a larger load, set a smaller pulse constant.
The external resistor sets the on-state current. Saia's note works this example: a 12 V supply, a 10 mA target, a 100 Ω series resistor inside the meter and about 0.2 V each across the optocoupler and the protection diode. The external resistor is (12 − 1 − 0.2 − 0.2) V ÷ 10 mA, which is approximately 1.06 kΩ. With the output on, the S0+ terminal is at 1.4 V. With the output off, a class A output can leak up to 2 mA. That leakage drops 2.1 V across 1.06 kΩ, so the terminal can be as low as 9.9 V. The receiver's threshold must be well inside the band from 1.4 V to 9.9 V.
On many DIN-rail meters, the S0 terminals are in the same terminal row as the mains conductors. Isolate the meter supply before you work on them.
Match the output to the counter input
Record these details before you select or wire a counter.
| Item | What to record |
|---|---|
| Output identity | Exact meter model, output option, terminals and configuration. A front-panel test LED can have a different pulse constant from the wired output. |
| Electrical limits | What supplies the sensing current, permitted voltage and current, input thresholds, on-state drop and off-state leakage |
| Reference and isolation | Polarity, signal return, isolation rating and any terminals shared between channels. Two terminals labelled common are not necessarily at the same potential. |
| Timing | Pulse width, minimum gap, maximum frequency, counted edge and any debounce or input filter setting |
| Meaning | Measured quantity, units per pulse or pulses per unit, and whether the output is import, export or another configured total |
Long cable adds capacitance. With a high-value pull-up, that capacitance slows the rising edge, and a short pulse can end before the signal crosses the threshold. The cable also picks up noise, which a contact input can count as pulses. Use the cable limits from the meter and counter manufacturers. Route and terminate the cable and its screen to the equipment instructions and the site's earthing design. If you add an interface or isolator, check its timing as well as its electrical ratings.
Check pulse width, gap and maximum rate
Pulse frequency is the number of complete pulses per second. Pulse width is the active time, and the gap is the inactive time before the next pulse. All three must be within the limits of the meter, any interface and the counter.
For a regular pulse train:
period in ms = 1000 ÷ frequency in Hz
pulse width + required minimum gap ≤ period
The manufacturer's maximum rate still applies, and input filtering can lower it. Schneider's PM3210 needs a pulse width of at least 50 ms and a gap at least as long as the pulse, so its output cannot run faster than 10 Hz.
For electricity, with a constant K in pulses/kWh and maximum power P in kW:
expected frequency = P × K ÷ 3600
For example, 36 kW at 1,000 pulses/kWh produces 10 Hz, with a 100 ms period. A 20 ms pulse leaves an 80 ms gap. A counter that needs a 10 ms minimum pulse, a 10 ms minimum gap and no more than 40 Hz passes these checks:
The same setting fails on a 200 kW feeder: 200 × 1,000 ÷ 3,600 is 55.6 Hz, above a 40 Hz input limit. The counter misses pulses at peak load and reads correctly at low load, so a spot check at night will not find the fault. At 100 pulses/kWh the same feeder gives 5.6 Hz.
Do the calculation at the maximum credible load, not the average. For water or gas, multiply the maximum flow per hour by pulses per unit, then divide by 3,600. A water meter with a 1 litre/pulse output at a peak flow of 25 m³/h gives 25,000 ÷ 3,600, which is 6.9 Hz. At 10 litres/pulse the same flow gives 0.69 Hz. The pulse meter scaling calculator does this calculation, shows the expected count in each reporting interval and shows when the counter rolls over.
A dedicated counter counts every edge and reports a total at each reporting interval. An ordinary digital input polled once a minute sees only the state at each poll and misses the pulses between polls. Confirm which method the acquisition device uses.
Convert pulse counts into consumption
Pulse constant (pulses per unit) is the reciprocal of pulse weight (units per pulse). Record the unit with the number.
| Meter setting | Conversion | Worked example |
|---|---|---|
| Pulses per kWh | Energy = count ÷ constant | 2,500 pulses ÷ 1,000 pulses/kWh = 2.5 kWh |
| Litres per pulse | Volume = count × weight | 75 pulses × 10 litres/pulse = 750 litres |
| Pulses per cubic metre | Volume = count ÷ constant | 240 pulses ÷ 100 pulses/m³ = 2.4 m³ |
Use the value configured for the terminal output. Some meters already include the transformer ratios: the PM3210 output represents primary energy, with the transformer ratios applied. If the monitoring system applies the ratio again, the total is too large by that ratio. The pulse meter scaling calculator takes pulses per kWh and shows the conversion and rate assumptions.
Keep the raw count and the scaling configuration. For a cumulative counter, calculate the difference between two readings, then convert that difference. Do not add successive cumulative totals together. A negative difference can be a rollover, a counter reset or a replaced counter. Check the counter's modulus before you treat it as a rollover.
To align a remote total with the meter display, record a meter reading and a counter reading at the same time. Add the scaled count difference to that baseline. A counter installed today has no record of the meter's earlier consumption.
At low consumption, few pulses arrive in a short interval. A 1 litre/pulse meter on a steady 2.5 litres/minute flow gives 2 counts in one minute and 3 in the next. The calculated flow steps between 2 and 3 litres/minute, and the total stays correct. Compare totals over a period long enough to include many pulses.
Pulse counting with EpiSensor
The ZPC pulse-counter family connects existing meter pulse outputs to the wireless Gateway. The current ZPC-2X datasheet, EPI-072-00, gives these values for the mains-powered models:
| Model or parameter | Published specification |
|---|---|
| ZPC-20 | One pulse channel |
| ZPC-22 | Two pulse channels |
| Minimum pulse width | 10 ms |
| Minimum gap between pulses | 10 ms |
| Maximum pulse rate | 40 Hz |
| Maximum input voltage | 35 V DC |
Apply all the timing limits together. Ten milliseconds on plus ten milliseconds off would allow 50 Hz, but the maximum is 40 Hz. A compliant S0 output fits inside these limits: its 30 ms pulse and gap are longer than 10 ms, its 16.7 Hz maximum is below 40 Hz and its 27 V class A maximum is below 35 V DC. These figures are for the mains-powered ZPC-2X only. The battery-powered ZPC-10 and ZPC-12 count up to 4 pulses a second, so the 1 litre/pulse water meter above, at 6.9 Hz, needs a mains model.
The datasheet does not give the input's sensing voltage, sensing current or switching threshold. It refers to the install sheet, which does. Read the install sheet for the installed model before you connect an S0 output, because S0 needs 10 mA (class A) or 2 mA (class B) in the on state from the receiver or an external supply.
The pulse meter application guide follows the count from the counter through the Gateway to Edge. Keep the pulse quantity, the conversion and the cumulative or interval meaning explicit at each step.
Third-party pulse counters and meter interfaces, including LoRaWAN pulse counters, are listed with their documented fields in the Device Directory.
Commissioning and troubleshooting
Before you accept a remote reading, make a known count change. Compare the source meter, the raw counter and the converted value over the same period. Then check the record at its final destination. A healthy radio link proves neither correct wiring nor correct scaling.
| Observed problem | Checks to make |
|---|---|
| No count change | Output enabled on the meter, enough consumption, sensing supply present where required, polarity and levels matched to the input |
| Extra counts | Contact bounce, interference, edge selection, duplicated received data |
| Missing counts at high load only | Maximum frequency, pulse width, gap and filtering at every interface, calculated at peak load |
| Correct raw count, wrong consumption | Pulse constant used as a weight, unit conversion, transformer ratio applied twice |
| Total jumps after an outage or restart | Retransmission, reset, rollover or replacement; how cumulative totals are processed |
The pulse meter commissioning and reconciliation guide covers the complete record: start and end totalisers, the controlled count test, scaling, persistence and recovery. Repeat the comparison after you change the meter's pulse configuration or replace the counter.
If the electrical match is uncertain, take the meter manual, terminal diagram, maximum rate and proposed cable route into System Builder. Those details decide whether a direct connection or an interface is needed.