The P1 port — how a smart meter puts its DSMR telegram out

The guide on large-consumer & indirect metering and the guide on dynamic load balancing for charging hubs both mention that control based on actual consumption is needed to make full use of a connection. For a connection with a Dutch smart meter, that information almost always flows through one specific interface: the P1 port. This guide covers what that port is, how it works, and what an installer runs into in practice.

What the P1 port is

The P1 port is a customer interface on the Dutch smart meter, based on NEN-EN-IEC 62056-21 mode D, that gives a local device (for example an energy monitor, an EV charge-point controller, or a home battery system) access to the current meter readings and the current power, without needing separate equipment requested from the grid operator.

Connector: an RJ12 (6P6C) modular connector.
Power supply: the port itself supplies +5V (up to roughly 250 mA) for a connected read-out module, on DSMR ≥ 4.0.
Galvanic separation: every pin of the P1 port is galvanically separated from the mains via opto-coupling, with over-voltage protection — a fault or short on the read-out side cannot directly propagate to the meter's own measurement chain.
Signal: a serial, open-collector output with a logically inverted signal (8N1), 115200 baud on DSMR ≥ 4.0 (9600 baud on older DSMR versions); reading it out requires a pull-up resistor between the data pin and the serial voltage used.

The DSMR telegram

As soon as the RTS (Request To Send) line on the P1 port is asserted, the meter sends an ASCII text message — the "telegram" — containing, among other things, the current meter readings (electricity import/export per tariff, gas meter reading if coupled), the current power drawn and fed back, and the voltage/current per phase. On DSMR versions older than 5.0 this telegram is sent every 10 seconds; from DSMR 5.0 onward it is sent every second, giving a finer-grained view of the instantaneous power.

Practical relevance for installers

EV charge points with dynamic load balancing: a charge-point controller that wants to match available charging power to the rest of the household load (see the guide on dynamic load balancing for charging hubs) typically reads the current power from the P1 port instead of requiring a separate current measurement on every circuit.
PV inverters and home battery systems: a system that wants to limit export (for example due to grid congestion, see the guide on grid congestion & battery storage) or that wants to store surplus in a battery first, uses the P1 telegram as the fastest available source for the current grid power.
Energy monitoring: a separate read-out module (often with Wi-Fi or Zigbee) on the P1 port gives insight into instantaneous consumption, without interfering with the grid operator's own measurement chain.

Practical consideration: no substitute for a certified measurement

The P1 telegram is meant for insight and control, not for billing or as a substitute for a certified, sealable current measurement where the installation requires one (for example a net-metering or supply agreement that requires a separate, calibrated meter). For control that is safety-relevant — for example a main-fuse protection scheme that switches circuits off based on the P1 power reading — the update rate of the telegram also needs to be accounted for: with an older meter using a 10-second interval, the control reacts noticeably more slowly to a sudden load increase than with a 1-second telegram.

Common mistakes

Assuming every P1 port updates at the same rate — a control scheme tested against a 1-second telegram (DSMR ≥ 5.0) can react too slowly on an older meter with a 10-second interval (DSMR < 5.0); this must be accounted for when designing a fast-reacting control scheme.
Not applying a pull-up resistor on the data line — the P1 port's open-collector output requires a pull-up to the serial voltage used; without this resistor no usable signal comes through.
Using the P1 read-out as a substitute for a mandatory certified measurement where the installation or supply agreement requires a separate, calibrated meter — the P1 telegram is meant for insight and control, not as a billing source.
Trying to connect multiple devices to the same P1 port simultaneously without a splitter designed for that purpose — the port is designed for one read-out module; an unsuitable parallel connection can degrade signal quality for both devices.