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IMPLEMENTATION

This protocol guide explains the operation of the Modbus

protocol per Modicon document PI-MBUS-300 Rev J.

The Magnetrol implementation of the Modbus protocol pro-

vides a standard form of digital communications. An effort

has been made to parallel current implementations to the

greatest extent possible, so that Magnetrol products com-

municate with existing Modbus masters.

Check compatibility carefully to ensure that the Model

300/301 is configured for the data format expected by the

host computer. Exceptions made because of the unique

requirements of the Magnetrol installation have been noted.

This is no guarantee, however, that the interpretation made

here will be the same as that followed by the Modbus master.

The Magnetrol implementation of the Modbus protocol pro-

vides for the passing of measured and calculated variables,

configuration information, and diagnostics in data registers.

Data is sent in these registers as floating point values, integer

values, numeric codes related to configuration lists, status

summary words (packed bits), or individual flags (single bits).

One master and up to 31 units may be multi-dropped on a

single RS-485 communication bus.

The Modbus functions implemented in Model 300/301 are

listed in Table 3.

CONFIGURATION

The Modbus port on the Model 300/301 must be configured

to establish communications. The keypad allows the user to

set the Magnetrol Modbus port to match the Modbus mas-

ter. See page 24 for configuration instructions.

Magnetrol addresses provide unique identification for the

host. This address may range from 1 to 247 and must be

unique. Each Model 300/301 only responds when a query

has been sent to its unique address by the host.

Magnetrol units support the Remote Terminal Unit (RTU)

mode of transmission. RTU provides improved error detec-

tion capabilities and higher throughput than the ASCII mode.

The RS-485 port must be configured for a transmission

speed (baud rate). Allowable values are 1200, 2400, 4800, or

9600 bits per second.

Function

Function

Information Type

MODBUS

Code

Nomenclature

01

Read

1-bit coils

Read coil status

02

Read

1-bit coils

Read input status

03

Read

16-bit register/integer

Read holding registers

04

Read

16-bit register/integer

Read input registers

05

Write

16-bit register/coils

Force single coil

06

Write

16-bit register

Preset single register

15

Write

16-bit register/coils

Force multiple coils

16

Write

16-bit register

Preset multiple registers

Exception:

Because the Model 300/301 does not distinguish between inputs and outputs, function codes 01 and 02 (as they apply to bits) and

function codes 03 and 04 (as they apply to numeric values) refer to the same data registers.

Table 3 Modbus Functions

MODBUS PROTOCOL