ReadsNode-RED Modbus Example

Read 32-bit Integers (INT32 / UINT32)

Energy counters, position feedback, and large totals often come back as 32-bit integers spread across two registers. This example reads two consecutive registers and interprets them as both UINT32 and INT32 in the two most common byte orders.

What you need

Node-RED v3+
node-red-contrib-modbus installed
Device with 32-bit integer registers (energy meter, counter, servo drive)

Flow Overview


Registers 20–21 on the wire:
  reg20 = 0x000F  (high word in AB-CD order)
  reg21 = 0x4240  (low word)

Combining 0x000F4240 → UINT32 = 1,000,000
                     → INT32  = 1,000,000 (positive, same)

If word-swapped (CD-AB): 0x4240000F → 1,111,638,031

Node-RED Flow JSON

[
  {"id":"inj5","type":"inject","name":"Poll 5s","repeat":"5","once":true,"wires":[["req5"]],"x":120,"y":500},
  {"id":"req5","type":"modbus-flex-getter","name":"Read 2 regs","dataType":"HoldingRegister","adr":20,"quantity":2,"server":"srv1","wires":[["fn5"],[]],"x":310,"y":500},
  {"id":"fn5","type":"function","name":"Decode INT32 / UINT32","func":"const [hi, lo] = msg.payload.data;
const buf = Buffer.alloc(4);
// AB-CD (big-endian): MSW in first register
buf.writeUInt16BE(hi, 0); buf.writeUInt16BE(lo, 2);
const uint32_ABCD = buf.readUInt32BE(0);
const int32_ABCD  = buf.readInt32BE(0);
// CD-AB (word-swap): MSW in second register
buf.writeUInt16BE(lo, 0); buf.writeUInt16BE(hi, 2);
const uint32_CDAB = buf.readUInt32BE(0);
const int32_CDAB  = buf.readInt32BE(0);
msg.payload = { raw:[hi,lo], 'UINT32-ABCD':uint32_ABCD,'INT32-ABCD':int32_ABCD,'UINT32-CDAB':uint32_CDAB,'INT32-CDAB':int32_CDAB };
return msg;","wires":[["dbg7"]],"x":540,"y":500},
  {"id":"dbg7","type":"debug","name":"32-bit values","active":true,"complete":"payload","wires":[],"x":750,"y":500},
  {"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: [15, 16960],
  "UINT32-ABCD": 1000000,
  "INT32-ABCD":  1000000,
  "UINT32-CDAB": 1111572495,
  "INT32-CDAB":  1111572495
}

Common Gotchas

Signed (INT32) and unsigned (UINT32) share the same raw bytes — only the interpretation differs.
A value of 4,294,967,295 as UINT32 is −1 as INT32 (two's complement).
The 'high word' vs 'low word' placement is device-specific — AB-CD means hi word at lower address.
Some Modicon PLCs use CD-AB (low word first) for all 32-bit types, including floats.
Always use Buffer.readUInt32BE / readInt32BE after assembling bytes — do not just multiply hi*65536+lo if the value could be negative.

Same in ModPackQT — in 30 seconds

ModPackQT's Data Type Converter decodes any register pair as UINT32, INT32, FLOAT32, or FLOAT64 in all byte orders simultaneously — no JavaScript required.

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