IntegrationNode-RED Modbus Example

Threshold Alert — Notify When a Register Exceeds a Limit

Trigger an alert when a Modbus register value crosses a threshold — an email, a Telegram message, or a webhook. This example monitors a temperature register and sends an alert when it exceeds 80°C, with a cooldown to avoid alert storms.

What you need

Node-RED v3+
node-red-contrib-modbus installed
An email or HTTP endpoint to receive the alert

Flow Overview


Poll every 30s → Read reg 0 → Scale (/10) → threshold > 80?
                                                     │
                                              5 min cooldown OK?
                                                     │
                                              Send webhook POST
                                              "ALERT: 82.4°C"

Node-RED Flow JSON

[
  {"id":"inj13","type":"inject","name":"Poll 30s","repeat":"30","once":true,"wires":[["req13"]],"x":120,"y":1300},
  {"id":"req13","type":"modbus-flex-getter","name":"Read temp","dataType":"HoldingRegister","adr":0,"quantity":1,"server":"srv1","wires":[["fn13"],[]],"x":310,"y":1300},
  {"id":"fn13","type":"function","name":"Check threshold","func":"const temp = msg.payload.data[0] / 10;
context.set('lastTemp', temp);
if (temp > 80) {
  const lastAlert = context.get('lastAlert') || 0;
  if (Date.now() - lastAlert > 300000) {  // 5 min cooldown
    context.set('lastAlert', Date.now());
    msg.payload = 'ALERT: Temperature ' + temp + '°C exceeds 80°C limit';
    msg.topic = 'temperature_alert';
    return msg;
  }
}
return null;","wires":[["http13"]],"x":510,"y":1300},
  {"id":"http13","type":"http request","name":"Send webhook","method":"POST","url":"https://your-webhook-url.example.com/alert","wires":[["dbg13"]],"x":720,"y":1300},
  {"id":"dbg13","type":"debug","name":"Alert sent","active":true,"wires":[],"x":900,"y":1300},
  {"id":"srv1","type":"modbus-client","name":"My Slave","clienttype":"tcp","tcpHost":"192.168.1.100","tcpPort":502,"unit_id":1}
]

To import: open Node-RED → Hamburger menu → Import → paste this JSON → Deploy.

Expected Output

// Only triggered when temp > 80°C and cooldown has passed:
POST https://your-webhook-url.example.com/alert
Body: "ALERT: Temperature 82.4°C exceeds 80°C limit"

Common Gotchas

Use context.get/set (not flow or global context) for per-node state so multiple alert nodes don't share cooldown.
Returning null from a function node drops the message — essential for the no-alert path.
Set a cooldown (5 minutes above) to avoid alert storms when temperature oscillates around the threshold.
Consider a hysteresis band: alert when > 80, clear when < 75 — add a second comparison for the 'all clear'.
For Telegram: use node-red-contrib-telegrambot instead of http request for richer message formatting.

Same in ModPackQT — in 30 seconds

ModPackQT Signal Bots do exactly this — monitor register values, apply conditions, and trigger webhooks, emails, or MQTT messages. Configure the threshold in the bot editor, no coding required.

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