Most industrial devices — energy meters, VFDs, PLCs, temperature sensors — speak Modbus RTU over RS485. That's great for local SCADA, but useless if you need data in a cloud dashboard 200 km away. This guide covers the full process: wiring RS485, mapping registers, configuring a 4G gateway, and getting clean JSON into your MQTT broker.

We'll use a Selec EM2M-1P-C-100A single-phase energy meter and a BusLog 4G gateway as the working example, but the concepts apply to any Modbus RTU device and cloud gateway.

What You Need

  • Selec EM2M-1P-C-100A energy meter (or any Modbus RTU slave device)
  • BusLog 4G gateway (or any 4G gateway with an RS485 port)
  • RS485 cable — twisted pair, shielded for runs > 10m
  • Device register map — included below for the EM2M
  • SIM card — any LTE Cat-1 carrier with a data plan

RS485 Wiring Basics

RS485 uses differential signaling over two wires (A and B). Simple, but easy to mess up in the field.

Standard Connection

Gateway          Slave Device
  A ──────────────── A
  B ──────────────── B
  GND ────────────── GND

For multiple devices, daisy-chain them — don't use star topology. RS485 is a bus:

Gateway ──── Device 1 ──── Device 2 ──── Device 3
                                           ┃
                                         120Ω
                                        (termination)

Wiring Rules

Rule Why
Always connect GND Floating ground = intermittent failures on long runs
Daisy-chain, not star Star topology causes signal reflections
Use shielded cable for > 10m Industrial environments have EMI everywhere
120Ω termination at last device for > 50m Prevents signal reflection on long buses
Max bus length: ~1200m at 9600 baud Beyond this, consider repeaters
Max 32 devices per bus (standard drivers) Some devices use 1/4 unit load, allowing 128

A/B Labelling Confusion

Different manufacturers label A and B differently. Some call A the non-inverting line (D+), others call it inverting (D−). If your first attempt at communication fails, swap A and B. It won't damage anything.

Understanding the Register Map

Before configuring anything, you need to know exactly which registers to read. This comes from your device's datasheet.

Modbus Register Types

Type Function Code Datasheet Prefix Access
Coils FC01 / FC05 0xxxx R/W
Discrete Inputs FC02 1xxxx Read only
Input Registers FC04 3xxxx Read only
Holding Registers FC03 / FC06 4xxxx R/W

Selec EM2M-1P-C-100A Register Map

The EM2M is a single-phase (1Ø-2W) DIN rail energy meter with RS485 Modbus RTU output. It's self-powered from 176–276V AC with a 100A max current rating — a common choice for single-phase sub-metering in commercial buildings.

All measurement data uses Input Registers (FC04), with each parameter stored as Float32 across 2 registers:

Parameter Datasheet Register Modbus Address Hex
Total Active Energy (kWh) 30000 0 0x00
Total Reactive Energy (kVArh) 30002 2 0x02
Import Reactive Energy 30004 4 0x04
Export Reactive Energy 30006 6 0x06
Apparent Energy (kVAh) 30008 8 0x08
Active Power (kW) 30010 10 0x0A
Reactive Power (kVAr) 30012 12 0x0C
Apparent Power (kVA) 30014 14 0x0E
Voltage L-N (V) 30016 16 0x10
Current (A) 30018 18 0x12
Power Factor 30020 20 0x14
Frequency (Hz) 30022 22 0x16
Max Demand Active Power 30024 24 0x18
Max Demand Reactive Power 30026 26 0x1A
Max Demand Apparent Power 30028 28 0x1C
Import Active Energy (kWh) 30028 28 0x1C
Export Active Energy (kWh) 30030 30 0x1E

The critical conversion: Datasheet register 30000 = Modbus address 0. For the EM2M, the offset is 30000 (not 30001 as in some other meters). Always verify against the actual datasheet.

The EM2M also exposes writable configuration via Holding Registers (FC03/FC06):

Parameter Register Range Default
Slave ID 40001 1–255 1
Baud Rate 40010 9600 / 19200 9600
Parity 40011 None / Odd / Even None
Stop Bit 40012 1 / 2 1

Byte Order — Float Swapped (CDAB)

Float32 values occupy 2 registers (4 bytes). The order these bytes are transmitted varies by manufacturer:

Byte Order Notation Common In
Mid-Big-Endian CDAB Selec meters, some Siemens devices
Big-Endian ABCD Schneider, ABB
Mid-Little-Endian BADC Rare
Little-Endian DCBA Some Asian meters

The Selec EM2M uses CDAB (Float Swapped) byte order. This means the two 16-bit registers are word-swapped compared to standard big-endian. If you configure ABCD and get values like 4.567e+18 or other garbage, switch to CDAB. This is the most common configuration mistake with Selec meters.

Example: A voltage reading of 230.5V in CDAB byte order:

Register 16 (high word): 0x6680   → bytes C, D
Register 17 (low word):  0x4366   → bytes A, B
Reassembled as ABCD: 43 66 80 00 → Float32 = 230.5

Connecting Selec EM2M to BusLog_4G — Step by Step

This section walks through the complete physical and software setup.

Step 1: Physical Wiring

The Selec EM2M has terminals 5–8 for RS485 communication. Connect to the BusLog 4G RS485 port as follows:

   Selec EM2M                    BusLog 4G
   (Terminals)                   (RS485 Port)
   ┌──────────┐                  ┌──────────┐
   │  5 (A+)  │────────────────│  A        │
   │  6 (B-)  │────────────────│  B        │
   │  7 (GND) │────────────────│  GND      │
   │  8 (GND) │  (spare GND)    │           │
   └──────────┘                  └──────────┘

   Power: EM2M is self-powered from 176–276V AC mains
          BusLog 4G needs 9–24V DC supply

Tip: The EM2M's terminal 8 is a second GND — useful if you're daisy-chaining to another device downstream.

Step 2: BusLog_4G Configuration

Configure the BusLog 4G to poll the EM2M registers. Here's the JSON device profile:

{
  "device_name": "EM2M_DB_Panel_1",
  "slave_id": 1,
  "baud_rate": 9600,
  "parity": "none",
  "stop_bits": 1,
  "poll_interval_sec": 10,
  "registers": [
    {
      "name": "voltage",
      "function_code": 4,
      "address": 16,
      "count": 2,
      "data_type": "float32",
      "byte_order": "CDAB",
      "unit": "V"
    },
    {
      "name": "current",
      "function_code": 4,
      "address": 18,
      "count": 2,
      "data_type": "float32",
      "byte_order": "CDAB",
      "unit": "A"
    },
    {
      "name": "active_power",
      "function_code": 4,
      "address": 10,
      "count": 2,
      "data_type": "float32",
      "byte_order": "CDAB",
      "unit": "kW"
    },
    {
      "name": "power_factor",
      "function_code": 4,
      "address": 20,
      "count": 2,
      "data_type": "float32",
      "byte_order": "CDAB",
      "unit": ""
    },
    {
      "name": "frequency",
      "function_code": 4,
      "address": 22,
      "count": 2,
      "data_type": "float32",
      "byte_order": "CDAB",
      "unit": "Hz"
    },
    {
      "name": "total_kwh",
      "function_code": 4,
      "address": 0,
      "count": 2,
      "data_type": "float32",
      "byte_order": "CDAB",
      "unit": "kWh"
    }
  ]
}

Key fields:

  • slave_id — must match the EM2M's configured address (default: 1, changeable via Holding Register 40001)
  • address — 0-indexed Modbus address. For the EM2M, subtract 30000 from the datasheet register number
  • count — always 2 for Float32 parameters
  • byte_order — CDAB for all Selec EM2M registers
  • poll_interval_sec — 10s is a good default; energy meters don't change faster than that

Step 3: What Happens on the Wire

When the gateway polls voltage from the EM2M (FC04, address 16, count 2), here's the actual Modbus RTU frame:

Request:  01 04 00 10 00 02 70 06
          ── ── ───── ───── ─────
          ID FC Addr  Count CRC16
          │  │  │     │     └─ CRC-16 (Modbus)
          │  │  │     └─ Read 2 registers (Float32)
          │  │  └─ Address 16 = 0x0010 (Voltage L-N)
          │  └─ FC04 = Read Input Registers
          └─ Slave ID 1

Response: 01 04 04 66 80 43 66 XX XX
          ── ── ── ─────────── ─────
          ID FC Len  Data(4B)  CRC16

The 4 data bytes 66 80 43 66 are in CDAB order. Reassemble to ABCD: 43 66 80 00 → Float32 = 230.5V. That's your voltage reading.

Step 4: Data in the Cloud

Once the gateway reads successfully, it pushes JSON over MQTT via 4G:

Topic:   siltech/{gateway_serial}/data
Payload:
{
  "ts": 1708243200,
  "device": "EM2M_DB_Panel_1",
  "voltage": 230.5,
  "current": 4.21,
  "active_power": 0.97,
  "power_factor": 0.99,
  "frequency": 50.01,
  "total_kwh": 14523.7
}

The gateway buffers data in internal memory during network outages and auto-uploads when 4G reconnects — no data loss even if cellular drops for hours.

From your MQTT broker, route this to any platform: Grafana, ThingsBoard, AWS IoT, Node-RED, or your own backend. It's standard JSON over standard MQTT.

BusLog IoT Platform showing Selec EM2M energy data

Gotchas / Debugging

These are the problems we see repeatedly across thousands of field deployments.

CRC Errors / No Response

Symptom Likely Cause Fix
TX LED blinks, RX doesn't Slave not responding Check slave ID, baud rate, parity. Try swapping A/B
Intermittent timeouts Loose wiring or EMI Check screw terminals, use shielded cable
Works on bench, fails on site Ground potential difference Connect GND wire between gateway and device

Wrong Values

Symptom Likely Cause Fix
Always 0 or 65535 Wrong register address Verify off-by-one: subtract 30000 for EM2M (not 30001)
Very large/small numbers Wrong byte order EM2M uses CDAB — not ABCD
Values half of expected Wrong data type (Int16 vs Float32) Check count — should be 2 for Float32
Negative when should be positive Signed vs unsigned mismatch Switch int16 to uint16

Bus Problems at Scale

  • More than 3-4 devices: Increase inter-frame delay (silent interval) to avoid collisions
  • Devices from different manufacturers: They may default to different baud rates — standardise everything to 9600/8N1 before connecting
  • Long cable runs (> 100m): Always terminate, always shield, always ground
  • Intermittent failures in summer: Thermal expansion loosens screw terminals. Use ferrules on wire ends

4G Connectivity Issues

  • No signal: Check antenna connection. The small PCB antenna works for most sites; use an external antenna for basements or metal enclosures
  • Connects but no data: Verify MQTT broker address and port. Check SIM data balance
  • Auto-APN not working: BusLog auto-detects Indian carriers. For private APNs, configure manually via the portal

Quick Reference / Checklist

Use this before every deployment:

Physical Setup

  • [ ] RS485 A, B, GND wired correctly (EM2M terminals 5, 6, 7 → BusLog A, B, GND)
  • [ ] Daisy-chain topology (not star)
  • [ ] 120Ω termination on last device (if cable > 50m)
  • [ ] Shielded cable for runs > 10m
  • [ ] SIM card inserted, antenna connected
  • [ ] Power supply: 9–24V DC to gateway, 176–276V AC to EM2M

Configuration

  • [ ] Slave ID matches EM2M setting (default: 1)
  • [ ] Baud rate 9600, parity None, stop bits 1 (EM2M defaults)
  • [ ] Register addresses are 0-indexed (subtract 30000 for EM2M)
  • [ ] Function code: FC04 for all measurement registers
  • [ ] Byte order set to CDAB (Float Swapped) for all EM2M registers
  • [ ] MQTT broker address and credentials configured

Verification

  • [ ] TX LED blinking = gateway polling
  • [ ] RX LED blinking = EM2M responding
  • [ ] Cloud dashboard shows expected values with correct units
  • [ ] Simulate a network drop and verify data buffering/recovery

Next Steps

Based on field experience from 4500+ BusLog deployments across India. Found an error or have a suggestion? Let us know — we update our guides based on real feedback.