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[Bug]: [Modbus] Support UDINT with least significant register first (little-endian word order)

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Evaluación

Dificultad
4/5
Tiempo estimado
3-5 días
Aptitud para principiantes
52/100
Tipo de issue
Nueva funcionalidad
Claridad
Bastante claro
Estado de actividad
Activo
Stack tecnológico
java
Área
backend, embedded-iot

Línea de trabajo

Start by reading the PLC4J Modbus tag decoding and payload byte-order code; the issue does not name specific files or tests. Check how UDINT currently handles two holding registers and whether byte order can be separated from register order. Done means a documented way to select least-significant-register-first produces 1431377 for the reported register values.

Escrito por el modelo de indexación a partir del texto del issue.

Descripción

What happened?

I would like to request support for decoding 32-bit unsigned integer values (UDINT) from Modbus holding registers when the least significant 16-bit register is transmitted first.

My Modbus energy meter exposes a 32-bit unsigned energy value across two consecutive holding registers. The meter returns the least significant register first, followed by the most significant register.

I can read both registers successfully using the UINT data type and manually reconstruct the 32-bit value. However, I have not been able to obtain the expected result directly using UDINT with LITTLE_ENDIAN.

Environment

Protocol: Modbus TCP

Driver: Apache PLC4X Modbus TCP

Device: Modbus energy meter

Unit Identifier: 19

Request timeout: 5000 ms

Default payload byte order: BIG_ENDIAN

Connection URL

String connectionUrl =
"modbus-tcp:tcp://10.10.121.55:502"
+ "?unit-identifier=19"
+ "&request-timeout=5000"
+ "&default-payload-byte-order=BIG_ENDIAN";

Steps to Reproduce

  1. Read the two holding registers as UINT

PlcReadRequest request = connection.readRequestBuilder()
.addTagAddress(
"energy",
"holding-register:29[0..1]:UINT{unit-id: 19}"
)
.build();

The response contains:

response = [55121, 21]
Tag: energy
Response code for energy: OK

The two 16-bit register values are:

First register: 55121

Second register: 21

  1. Reconstruct the unsigned 32-bit value manually

The expected result is calculated by placing the second register in the upper 16 bits and the first register in the lower 16 bits:

long energyValue =
((long) 21 << 16) | 55121L;

Result:

Assembled Energy Value: 1431377

This produces the expected energy value.

  1. Attempt to read the value directly as UDINT

I am unable to obtain the same expected value by reading the two registers directly as UDINT with LITTLE_ENDIAN byte order.

I would like PLC4X to support decoding this register arrangement directly, without requiring applications to read individual registers and reconstruct the value manually.

Expected Behavior

PLC4X should support decoding a 32-bit unsigned integer from two Modbus holding registers when the least significant register is first.

For the example above:

Register 1 (LSW) = 55121
Register 2 (MSW) = 21

Expected UDINT value = 1431377

The driver should provide a documented and supported way to select the register/word order independently of the byte order within each 16-bit register, where necessary.

Actual Behavior

The two registers can be read successfully as UINT, and the expected value can be calculated manually.

However, I cannot obtain the expected result directly using UDINT with LITTLE_ENDIAN.

Requested Enhancement

Please clarify whether the Modbus driver supports 32-bit unsigned integer decoding with least significant register first.

If this arrangement is not currently supported, please consider adding support for configurable 16-bit register/word ordering for multi-register data types such as:

UDINT — unsigned 32-bit integer

DINT — signed 32-bit integer

REAL — 32-bit floating-point value

Other supported multi-register data types, where applicable

The configuration should distinguish between:

Byte order: the ordering of bytes within an individual 16-bit register.

Word/register order: the ordering of the two 16-bit registers that make up a 32-bit value.

These settings should be independently configurable where the protocol and driver implementation permit it.

Additional Information

The Modbus connection and read request are successful, and the response code is OK.

The issue concerns decoding the two register values into the expected 32-bit unsigned integer rather than reading the registers themselves.

Version

v1.0.0

Programming Languages
  • plc4c
  • plc4go
  • plc4j
  • plc4net
  • plc4py
Protocols
  • AB-Ethernet
  • ADS /AMS
  • BACnet/IP
  • C-Bus
  • CANopen
  • EtherNet/IP
  • Firmata
  • IEC-69870
  • KNXnet/IP
  • Modbus
  • OPC-UA
  • PLC4X
  • Profinet
  • S7
  • Simulated
  • SLMP
  • UMAS
Lenguaje dominante
Java
Estrellas
1.8k
Forks
512
Merge medio
2 d 2 h
PR fusionados (30 d)
34

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