使用Modbus协议与Micro Motion变送器

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"该文档是关于如何使用Modbus®协议配置、操作和维护支持Modbus协议的Micro Motion®变送器的手册。手册由Micro Motion Inc.于2004年发布，提供了技术支持联系方式，并列出了书中涵盖的内容，包括LF系列变送器的介绍以及Modbus接口的相关详细信息。" 在工业自动化领域，Modbus协议是一种广泛应用的通信协议，它允许不同的设备之间进行数据交换，特别是PLC（可编程逻辑控制器）和各种仪表之间。Micro Motion®变送器是高精度流量和密度测量设备，使用Modbus协议能够实现与上位机或控制系统进行数据交互，便于系统集成和远程监控。 **第1章：开始之前** 在这一章中，读者会了解到该手册的主要内容，即如何利用Modbus协议配置和管理Micro Motion®变送器。手册指出，仅部分特定型号的变送器支持Modbus协议，例如LF系列。此外，它还说明了如何有效使用手册，以及LF系列变送器的特点。同时，手册强调了一些不在其涵盖范围内的内容，并提供了客户服务的联系方式，以便用户在遇到问题时寻求帮助。 **第2章：Modbus接口** 这一章详细介绍了Modbus接口的工作原理。首先，它解释了什么是Modbus协议的“轮询地址”，这是设备在网络中被识别和通信的地址。接下来，重点讨论了变送器的寄存器和数据类型，这是Modbus协议的核心部分。 - **2.3.1 变送器寄存器和数据类型** - **输入寄存器（Input Registers）**：这些寄存器只读，用于从变送器获取测量值或其他状态信息。 - **保持寄存器（Holding Registers）**：双向读写，用于设置变送器的参数或存储测量数据。 - **数据类型**：包括整数、浮点数、布尔值等，它们定义了寄存器中数据的格式和含义。通过这些寄存器，用户可以读取变送器的实时测量数据，如流量、密度，也可以设置变送器的工作模式、报警阈值等配置参数。这个手册为用户提供了全面的指南，使他们能够有效地利用Modbus协议与Micro Motion®变送器进行通信，实现自动化系统的集成和高效运行。对于工业自动化领域的工程师和技术人员来说，理解和掌握这部分知识至关重要，因为它可以帮助他们更便捷地监控和控制生产过程中的关键参数。

6 Using Modbus Protocol with Micro Motion Transmitters

Modbus Interface continued

The RFT9739 transmitter must receive and send floating-point data in the following byte order:

2–3–0–1

MVD transmitters can receive and send floating-point data in the following byte orders:

0–1–2–3

2–3–0–1

1–0–3–2

3–2–1–0

For information on configuring floating-point byte order, see Section 4.4.

ASCII registers

Modbus functions 3, 4, 6, and 16 are used to read and write ASCII registers (see Section 2.4.2). String

values are padded to 4, 8, 16, or 32 characters. The space character is used to pad data read from the

transmitter, and don’t care is used to pad data sent to the transmitter.

When reading or writing ASCII registers, the first register must always be read or written. You can

read or write the first register, the first two registers, the first three registers, and so on. The registers

must be read or written with one command. More than one character string can be read or written in a

single command, if the ASCII registers are consecutive.

Note: To read several non-consecutive ASCII register sets with a single command, use slot addresses

(see Chapter 14).

2.4 Message framing

Modbus messages are structured in frames. In general, each frame contains an address field, a

function field, a data function field, and an error check field. Frames are structured and transmitted

differently depending on the data transmission mode (see Section 2.5).

There are three frame types:

• Query frames are used to address a single network device. The network device responds with a

response frame. The combination of one query frame and one response frame is called a

query/response message.

• Broadcast frames are used to address all devices on the network. The devices do not respond.

Each broadcast frame constitutes one broadcast message. Any frame with a value of 0 in the

address field is a broadcast frame.

Only Modbus function codes 5, 6, 8, 15, and 16 are valid in a broadcast message. Every slave

device on a multidrop network will receive a broadcast message and perform the requested

action, and no slave will respond, which is why no read functions are supported in a broadcast

message.

Because of its use in broadcast messages, address 0 is not useful as a Modbus polling address.

2.4.1 Address field

In a query frame, the address field contains a transmitter's polling address.

In a response frame, the address field contains the polling address of the responding device.

In a broadcast frame, the address field contains a 0, which tells the network devices not to reply.

Using Modbus Protocol with Micro Motion Transmitters 7

Modbus Interface continued

Modbus Interface CommunicationsSensor and Transmitter InformationBefore You Begin

2.4.2 Function field

The function field contains a Modbus function code. Micro Motion’s Modbus interface uses a subset

of function codes supported by all Modbus hosts. The function codes used by Micro Motion’s

Modbus interface are listed and defined in Table 2-2

In Table 2-2:

• Functions 01 and 02 as used by Micro Motion perform the same processing, and are

interchangeable.

• Functions 03 and 04 as used by Micro Motion perform the same processing, and are

interchangeable.

In a query frame or broadcast frame, the function code specifies a read command, write command, or

diagnostic command to be applied to the register specified in the data field.

In a response frame, the function code is used to verify the device's response to the command:

• If no error occurred, the transmitter echoes the original function code. The data field contains

the start register and the value sent, or the start register and the function number. See

Section 2.4.3.

• If an error occurred, the most significant bit in the function field is set. The data field contains

an exception response explaining any errors encountered processing the command.

Table 2-2 Function code definitions

Function

code

Command

type Description Explanation

Read Read coil status • Read status of one coil or consecutive coils

Read Read integer, float, or

ASCII registers

• Read integer values in one holding register or consecutive holding registers

• Read floating-point values in one register pair or consecutive register pairs

• Read character strings in consecutive ASCII registers

05 Write Set coil • Set coil to a specified ON or OFF state

06 Write Load register • Write an integer value to an integer register

• Write a single-precision IEEE 754 floating-point value to a register pair

(1) Supported only by RFT9739 transmitters, MVD Direct Connect systems, and other MVD systems if the Modbus wiring is direct to

the core processor.

Diagnostic Read exception status Read ON/OFF status of status bits that comprise the high-order byte of

status. They include:

• Bit #0 (E)EPROM checksum failure

• Bit #1 RAM diagnostic failure

• Bit #2 Sensor failure

• Bit #3 Temperature sensor failure

• Bit #4 Input overrange

• Bit #5 Frequency overrange

• Bit #6 Transmitter not configured

• Bit #7 Real-time interrupt failure

See Chapter 21 for more information.

Diagnostic Loopback diagnostic • Send test message to transmitter to evaluate communication processing

15 Write Set multiple coils • Set consecutive coils to an ON or OFF state

16 Write Load multiple registers • Write integer values to consecutive holding registers

• Write single-precision IEEE 754 floating-point values to consecutive register

pairs

• Write a single-write multiple-character string to one ASCII register set (four

to eight consecutive ASCII registers)

Read Report device I.D. • Report AA

hex

device type and ON operating status

10 Using Modbus Protocol with Micro Motion Transmitters

Modbus Interface continued

Function code 17

If a device detects errors while executing a command issued in a query, the most significant bit in the

response function field is set, and the data field contains an exception response that explains why the

device cannot execute the command.

Table 2-3 lists and describes all Modbus exception responses used by the Micro Motion Modbus

interface.

2.4.4 Error check field

The error check field allows the Modbus host and other network devices to check a message for data

transmission errors. Electrical noise or other interference can cause a message to change during

transmission from one unit to another unit in the network. Error checking ensures that the host or

other network device does not react to messages that have changed during data transmission. See

Section 2.6 for more information on error checking.

2.5 Data transmission modes

Modbus protocol uses either ASCII (7-bit) or RTU (8-bit) data transmission modes. In RTU mode,

messages consist of 8-bit binary characters, while in ASCII mode a message consists of 7-bit ASCII

characters. To obtain the ASCII mode representation of a message, each 8-bit RTU character is

divided into two 4-bit parts: a high order part and a low order part. Each of these 4-bit parts is then

converted to its hexadecimal equivalent, and the 7-bit ASCII codes corresponding to the hexadecimal

numbers are transmitted. ASCII mode requires approximately twice as many characters as RTU mode

because a single 8-bit RTU character equals two 7-bit ASCII characters.

Modbus protocol also has several levels of error checking, including automatic redundancy checks.

The data transmission mode determines the form of the redundancy checks: ASCII mode relies on

Longitudinal Redundancy Checks (LRC); RTU mode relies on Cyclic Redundancy Checks (CRC).

Table 2-4 summarizes these differences.

Query

Address Function Error check

Response

Address Function Byte count

(1) Byte count is the number of bytes in the slave ID and run indicator status fields. Each field contains

one byte, so byte count is always 2.

Slave ID Run indicator status Error check

Table 2-3 Exception response definitions

Exception

response Description Explanation

01 Illegal function The received message function is not an allowable action for the transmitter

02 Illegal data address The address referenced in the data field is not an allowable address for the location

03 Illegal data value The value referenced in the data field is not allowed in the addressed location

06 Busy, rejected

message

The device received the message without error, but is engaged in processing a long-duration

program command

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