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UNPACKVALUE and PACKVALUE convert between typed values — integers, floats, text — and the raw registers or bytes of a collection. They do not touch the bus, so they work in every script: on Modbus register blocks, on TCP and UDP payloads in a Packet parser listener script, or in a smart rule. The data type is one of the Modbus data types. Both functions need Core 2026.1, build 39001 or newer, and an app of build 39016 or newer.

UNPACKVALUE#

UNPACKVALUE reads one typed value from a register collection (collection<uint16>) or a byte collection (collection<uint8>) at the given offset.

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UNPACKVALUE(collection, offset, dataType)
UNPACKVALUE(collection, offset, String, length)

Returns — a number; bool for Bool; int64 or uint64 for the 64-bit integer types; a string for String.

Remarks — RECEIVEDMSG.PAYLOAD is a blob and is not accepted. In a TCP or UDP listener script use RECEIVEDBYTES, which is a byte collection, or convert text with TOBYTES.

Example — values from one Modbus block read

collection<uint16> data := MODBUSRBLOCK(H, 100, 4);
VAR power := UNPACKVALUE(data, 0, BigEndianFloat);
VAR state := UNPACKVALUE(data, 2, Uint16);
VAR name := UNPACKVALUE(data, 0, String, 2);

Example — a number from received bytes

collection<uint8> frame := BYTECOLLECTION("01 02 00 FA");
UNPACKVALUE(frame, 2, Uint16);
# Result: 250

collection<uint8> frame := BYTECOLLECTION("FA 00 00 00");
UNPACKVALUE(frame, 0, LittleEndianUint32);
# Result: 250

Errors — UNPACKVALUE: offset N + M element(s) exceeds collection length L; UNPACKVALUE: parameter 1 must be a Collection<UInt16> or Collection<UInt8>; UNPACKVALUE: String data type must have specified length - number of registers to read.

PACKVALUE#

PACKVALUE writes one typed value into a register or byte collection at the given offset and returns the same, changed collection.

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PACKVALUE(collection, offset, dataType, value)

Remarks

• The collection is changed in place; the return value is the same collection.
• String is not supported.
• Integer types round the value to a whole number. A value outside the range of the type, or NaN written to an integer type, stops the script.

Example — build a register block and write it in one transaction

collection<uint16> block := UINT16COLLECTION(4);
PACKVALUE(block, 0, BigEndianFloat, 21.5);
PACKVALUE(block, 2, Uint16, 100);
MODBUSWBLOCK(H, 200, block);

Example — bytes for a TCP or UDP command

collection<uint8> frame := BYTECOLLECTION(4);
PACKVALUE(frame, 0, Uint16, 0x0102);
PACKVALUE(frame, 2, LittleEndianUint16, 0x0304);
SENDDATA(frame);
# frame is now 01 02 04 03

Errors — PACKVALUE: String data type is not supported; PACKVALUE: Can’t write double.NaN to modbus register; Overflow when writing X as UInt16.

Byte order in collections#

The same data type names describe the order of registers in a collection<uint16> and the order of bytes in a collection<uint8>.

Remarks — In a byte collection LittleEndian already reverses all bytes, so adding ByteSwap to it changes nothing.

See also — Modbus data types, MODBUSRBLOCK, RECEIVEDBYTES.