A meter that can only be read by a person walking up to it with a torch is a 20th-century meter. Modern meters communicate — locally for commissioning and testing, and remotely for automated meter reading. This page covers every physical interface you will encounter.

The Optical Port (IEC 62056-21)

Every meter has a circular optical port on its front. A technician clips an optical probe onto it. The probe converts IR light pulses to RS-232 signals for a laptop or handheld device.

OPT1: terminal_strip label="Optical Port" terminals="1:TX@left,2:RX@left,3:GND@left"
PROBE: terminal_strip label="Optical Probe" terminals="1:RX@right,2:TX@right,3:GND@right"

OPT1.1 --> PROBE.1 wire=001 color=RD
OPT1.2 --> PROBE.2 wire=002 color=GN
OPT1.3 --> PROBE.3 wire=003 color=BK

Used for: commissioning, calibration, local parameter programming, manual meter reading.
Speed: 300 baud (initial handshake) → up to 115,200 baud after negotiation.

RS-485

RS-485 is a differential two-wire bus that can connect many meters in a daisy-chain to a single Data Concentrator Unit (DCU). It is robust against electrical noise — ideal for industrial environments.

M1: terminal_strip label="Meter 1" terminals="1:A@right,2:B@right,3:GND@right"
M2: terminal_strip label="Meter 2" terminals="1:A@right,2:B@right,3:GND@right"
M3: terminal_strip label="Meter 3" terminals="1:A@right,2:B@right,3:GND@right"
DCU: terminal_strip label="Data Concentrator" terminals="1:A@left,2:B@left,3:GND@left"

M1.1 --> DCU.1 wire=A color=BN size=0.75mm2
M1.2 --> DCU.2 wire=B color=BU size=0.75mm2
M2.1 --> DCU.1 wire=A color=BN size=0.75mm2
M2.2 --> DCU.2 wire=B color=BU size=0.75mm2
M3.1 --> DCU.1 wire=A color=BN size=0.75mm2
M3.2 --> DCU.2 wire=B color=BU size=0.75mm2

• Max distance: 1200 m
• Max devices: 32 (standard driver), up to 256 with repeaters
• Speed: up to 10 Mbps (typically 9600–115200 bps in metering)

Power Line Communication (PLC)

PLC injects a high-frequency data signal onto the mains power wires themselves. No extra cables needed — the power line is the network.

Two dominant standards:

• G3-PLC — narrowband, robust, used in India (BEE mandate), Europe
• PRIME — faster, used widely in Spain

Frequency band: 3–500 kHz (CENELEC A/B/C bands). Data rate: 2–300 kbps depending on standard.

RF Mesh

RF mesh creates a self-healing wireless network between meters. Each meter is a node that can relay data from its neighbours.

Technologies: Wi-SUN (IEEE 802.15.4g), Zigbee, LoRa (star topology), proprietary sub-GHz.

Advantage: no existing infrastructure needed. Disadvantage: requires dense deployment for mesh connectivity.

GPRS / 4G / NB-IoT

Some meters, especially in remote or rural areas, communicate directly to the cloud via a SIM card. No DCU needed.

Comparison at a Glance

Key Takeaway

No single interface wins in all situations. Most large AMI deployments use PLC or RF as the primary last-mile interface, with cellular as a fallback for meters that cannot reach the mesh. The optical port is always present for field servicing, regardless of what remote interface the meter uses.