Initial implementation: MQTT-based energy meter subscriber

Subscribes to Eastron SDM630 power meter data via MQTT broker,
decodes Modbus RTU frames, and writes readings to InfluxDB.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

requester.go

@@ -0,0 +1,112 @@
+package main
+
+import (
+	"encoding/binary"
+	"encoding/json"
+	"fmt"
+	"sync/atomic"
+	"time"
+
+	mqtt "github.com/eclipse/paho.mqtt.golang"
+)
+
+// ── CRC-16 / Modbus ───────────────────────────────────────────────────────────
+
+// crc16 computes the Modbus RTU CRC-16 (poly 0xA001, initial value 0xFFFF).
+// The result must be appended as [low_byte, high_byte].
+func crc16(data []byte) uint16 {
+	crc := uint16(0xFFFF)
+	for _, b := range data {
+		crc ^= uint16(b)
+		for range 8 {
+			if crc&1 != 0 {
+				crc = (crc >> 1) ^ 0xA001
+			} else {
+				crc >>= 1
+			}
+		}
+	}
+	return crc
+}
+
+// ── Frame building ────────────────────────────────────────────────────────────
+
+// buildFC4Frame builds a Modbus RTU FC4 (Read Input Registers) request frame.
+//
+//	[slave, 0x04, regAddr_hi, regAddr_lo, count_hi, count_lo, crc_lo, crc_hi]
+//
+// regAddr and count are in register (word) units.
+func buildFC4Frame(slave byte, regAddr, count uint16) []byte {
+	frame := make([]byte, 6)
+	frame[0] = slave
+	frame[1] = 0x04
+	binary.BigEndian.PutUint16(frame[2:], regAddr)
+	binary.BigEndian.PutUint16(frame[4:], count)
+	crc := crc16(frame)
+	return append(frame, byte(crc), byte(crc>>8)) // little-endian CRC
+}
+
+// ── MQTT payload ──────────────────────────────────────────────────────────────
+
+type requestPayload struct {
+	Data struct {
+		ModbusFrame struct {
+			Value []int  `json:"value"`
+			Unit  string `json:"unit"`
+		} `json:"modbus_frame"`
+	} `json:"data"`
+	Timestamp struct {
+		Incarnation int64 `json:"incarnation"`
+		SystemTime  int64 `json:"system_time"`
+	} `json:"timestamp"`
+}
+
+// ── Requester ─────────────────────────────────────────────────────────────────
+
+// Requester publishes Modbus FC4 read requests to the MQTT broker.
+// Topic pattern: <topicPrefix>/<segment>/0x<slave_hex>
+type Requester struct {
+	client      mqtt.Client
+	topicPrefix string
+	incarnation atomic.Int64
+	startTime   time.Time
+}
+
+func NewRequester(client mqtt.Client, topicPrefix string) *Requester {
+	return &Requester{
+		client:      client,
+		topicPrefix: topicPrefix,
+		startTime:   time.Now(),
+	}
+}
+
+// ReadInputRegisters sends an FC4 read request for `count` registers (words)
+// starting at `regAddr` on the given slave within the named segment.
+// count=2 reads one float32 value; use a multiple of 2 for more values.
+func (r *Requester) ReadInputRegisters(segment string, slave byte, regAddr, count uint16) error {
+	frame := buildFC4Frame(slave, regAddr, count)
+
+	var p requestPayload
+	p.Data.ModbusFrame.Unit = "RAW"
+	p.Data.ModbusFrame.Value = bytesToInts(frame)
+	p.Timestamp.Incarnation = r.incarnation.Add(1)
+	p.Timestamp.SystemTime = time.Since(r.startTime).Nanoseconds()
+
+	payload, err := json.Marshal(p)
+	if err != nil {
+		return fmt.Errorf("marshalling request: %w", err)
+	}
+
+	topic := fmt.Sprintf("%s/%s/0x%02x", r.topicPrefix, segment, slave)
+	token := r.client.Publish(topic, 0, false, payload)
+	token.Wait()
+	return token.Error()
+}
+
+func bytesToInts(b []byte) []int {
+	ints := make([]int, len(b))
+	for i, v := range b {
+		ints[i] = int(v)
+	}
+	return ints
+}