ReadsNode-RED Modbus Example

Read FLOAT32 Across All Four Byte Orders

Reading a 32-bit float is straightforward once you know the byte order. The problem is every manufacturer picks a different one — and some datasheets don't tell you. This example reads 2 consecutive holding registers and decodes them as a FLOAT32 in all four common byte orders so you can identify the correct one.

What you need

Node-RED v3+
node-red-contrib-modbus installed
A device that exposes at least one FLOAT32 value (e.g. temperature or voltage)

Flow Overview


┌─────────┐   ┌───────────────────────┐   ┌──────────────────────────────────────────┐
│ Inject  │──▶│ Read 2 Holding Regs   │──▶│ Function: try all 4 byte orders          │
│         │   │ addr:10, qty:2        │   │  AB-CD → float (big-endian)              │
└─────────┘   └───────────────────────┘   │  CD-AB → float (word-swap)              │
                                          │  BA-DC → float (mixed)                  │
                                          │  DC-BA → float (little-endian)          │
                                          └──────────────────────────────────────────┘
                                                          ▼
                                              Debug: { raw:[…], AB-CD: …, … }

Node-RED Flow JSON

[
  {"id":"inj4","type":"inject","name":"Poll once","once":true,"wires":[["req4"]],"x":120,"y":400},
  {"id":"req4","type":"modbus-flex-getter","name":"Read 2 regs","dataType":"HoldingRegister","adr":10,"quantity":2,"server":"srv1","wires":[["fn4"],[]],"x":310,"y":400},
  {"id":"fn4","type":"function","name":"Try all byte orders","func":"const d = msg.payload.data;
const b = Buffer.alloc(4);
b.writeUInt16BE(d[0], 0); b.writeUInt16BE(d[1], 2);
const AB_CD = b.readFloatBE(0);
b.writeUInt16BE(d[1], 0); b.writeUInt16BE(d[0], 2);
const CD_AB = b.readFloatBE(0);
b.writeUInt16BE(d[0], 0); b.writeUInt16BE(d[1], 2);
const BA_DC = b.swap16().readFloatBE(0);
const buf2 = Buffer.alloc(4);
buf2.writeUInt16BE(d[1], 0); buf2.writeUInt16BE(d[0], 2);
buf2.swap16();
const DC_BA = buf2.readFloatBE(0);
msg.payload = { raw: d, 'AB-CD': AB_CD, 'CD-AB': CD_AB, 'BA-DC': BA_DC, 'DC-BA': DC_BA };
return msg;","wires":[["dbg6"]],"x":530,"y":400},
  {"id":"dbg6","type":"debug","name":"All interpretations","active":true,"complete":"payload","wires":[],"x":740,"y":400},
  {"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

msg.payload = {
  raw: [17224, 0],          // registers 10 and 11
  "AB-CD": 200.0,          // ← most likely correct if device is big-endian
  "CD-AB": 2.8e-43,
  "BA-DC": 3.1e-38,
  "DC-BA": 1.4e-45
}
// The value that makes physical sense is your byte order.

Common Gotchas

The correct byte order is the one that produces a physically plausible value — pick the one closest to what the sensor should read.
Registers are read MSB-first within each 16-bit word regardless of byte order — that is fixed.
Some vendors document byte order as '3412' or '1234' — 3412 = AB-CD, 2143 = CD-AB, 4321 = DC-BA, 1234 = BA-DC.
Buffer.swap16() swaps bytes within each 16-bit pair — use it to implement BA-DC and DC-BA.
Use float32 with the SAME byte order for all registers on the same device (they don't mix).

Same in ModPackQT — in 30 seconds

ModPackQT's Float-to-Register tool decodes any pair of register values as FLOAT32 in all four byte orders side by side, so you can visually confirm the right order in one click.

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