Thomas Klaehn
066fa5ca51
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>
113 lines
3.5 KiB
Go
113 lines
3.5 KiB
Go
package main
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import (
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"encoding/binary"
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"encoding/json"
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"fmt"
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"sync/atomic"
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"time"
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mqtt "github.com/eclipse/paho.mqtt.golang"
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)
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// ── CRC-16 / Modbus ───────────────────────────────────────────────────────────
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// crc16 computes the Modbus RTU CRC-16 (poly 0xA001, initial value 0xFFFF).
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// The result must be appended as [low_byte, high_byte].
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func crc16(data []byte) uint16 {
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crc := uint16(0xFFFF)
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for _, b := range data {
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crc ^= uint16(b)
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for range 8 {
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if crc&1 != 0 {
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crc = (crc >> 1) ^ 0xA001
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} else {
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crc >>= 1
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}
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}
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}
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return crc
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}
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// ── Frame building ────────────────────────────────────────────────────────────
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// buildFC4Frame builds a Modbus RTU FC4 (Read Input Registers) request frame.
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//
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// [slave, 0x04, regAddr_hi, regAddr_lo, count_hi, count_lo, crc_lo, crc_hi]
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//
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// regAddr and count are in register (word) units.
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func buildFC4Frame(slave byte, regAddr, count uint16) []byte {
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frame := make([]byte, 6)
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frame[0] = slave
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frame[1] = 0x04
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binary.BigEndian.PutUint16(frame[2:], regAddr)
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binary.BigEndian.PutUint16(frame[4:], count)
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crc := crc16(frame)
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return append(frame, byte(crc), byte(crc>>8)) // little-endian CRC
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}
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// ── MQTT payload ──────────────────────────────────────────────────────────────
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type requestPayload struct {
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Data struct {
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ModbusFrame struct {
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Value []int `json:"value"`
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Unit string `json:"unit"`
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} `json:"modbus_frame"`
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} `json:"data"`
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Timestamp struct {
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Incarnation int64 `json:"incarnation"`
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SystemTime int64 `json:"system_time"`
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} `json:"timestamp"`
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}
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// ── Requester ─────────────────────────────────────────────────────────────────
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// Requester publishes Modbus FC4 read requests to the MQTT broker.
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// Topic pattern: <topicPrefix>/<segment>/0x<slave_hex>
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type Requester struct {
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client mqtt.Client
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topicPrefix string
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incarnation atomic.Int64
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startTime time.Time
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}
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func NewRequester(client mqtt.Client, topicPrefix string) *Requester {
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return &Requester{
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client: client,
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topicPrefix: topicPrefix,
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startTime: time.Now(),
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}
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}
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// ReadInputRegisters sends an FC4 read request for `count` registers (words)
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// starting at `regAddr` on the given slave within the named segment.
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// count=2 reads one float32 value; use a multiple of 2 for more values.
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func (r *Requester) ReadInputRegisters(segment string, slave byte, regAddr, count uint16) error {
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frame := buildFC4Frame(slave, regAddr, count)
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var p requestPayload
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p.Data.ModbusFrame.Unit = "RAW"
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p.Data.ModbusFrame.Value = bytesToInts(frame)
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p.Timestamp.Incarnation = r.incarnation.Add(1)
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p.Timestamp.SystemTime = time.Since(r.startTime).Nanoseconds()
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payload, err := json.Marshal(p)
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if err != nil {
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return fmt.Errorf("marshalling request: %w", err)
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}
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topic := fmt.Sprintf("%s/%s/0x%02x", r.topicPrefix, segment, slave)
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token := r.client.Publish(topic, 0, false, payload)
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token.Wait()
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return token.Error()
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}
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func bytesToInts(b []byte) []int {
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ints := make([]int, len(b))
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for i, v := range b {
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ints[i] = int(v)
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}
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return ints
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}
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