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ID

25463

Products

Programmable Controllers

MicroLogix 1200 (1762)

MicroLogix 1500 (1764)

Category

General

Date Created

10/30/2003

Last Updated

02/05/2008

Access Level

Everyone

Prev. TN#

A62452563

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Quick Start Modbus Master Protocol Example For The MicroLogix 1200/1500.

Question

Quick Start Modbus Master Protocol example for the MicroLogix 1200/1500

Answer

This document can be used as a "Quick Start"for help setting up the

Modbus RTU Master protocol for the MicroLogix 1200/1500 processors

The following illustrates a ML1500 LRP processor (configured for Modbus RTU Master) initiating

a message via Modbus protocol to a ML1500 LSP (configured as a Modbus RTU Slave). Since both

ports on the ML1500 units used in this example are both RS232 ports the 1761-NET-AIC's are not

required for this specific example but are referenced to point out that the above scenario has

been tes ted. In order to connect ML1200/1500 processor to a RS-485 multi-drop network an

RS-232 to RS-485 converter, such as the 1761-NET-AIC, would be required.

NOTE: Publication 1762-RM001E (MicroLogix Instruction Set Reference Manual) was updated and

includes information on Modbus RTU Master channel configuration and MSG configuration.

NOTE: The Modbus RTU Master Driver is available on ML1200 processors (Series C FRN 8 or

later) and ML1500 processors (Series C FRN 9 or later).

NOTE: RSLogix500 version 6.10.10 is required to configure a Modbus RTU Master in either the

ML1200 or ML1500 processors.

NOTE: The Modbus RTU Slave Protocol is available on ML1200 processors (Series B FRN 3 or

later) and ML1500 processors (Series B FRN 4 or later).

NOTE: Only the Modbus RTU Master is capable of messaging. Modbus RTU Slave cannot initiate

any MSG's. Modbus RTU Slaves can only respond to a Modbus RTU Master.

NOTE: 1761-NET-AIC's have been tested with the Modbus protocol and do work. However we

cannot guaranty that the AIC+'s will be compatible with other 3rd party converters on the same

network.

Modbus RTU Master CH1 Configuration

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NOTE: There is no node number designated on the Modbus RTU Master.

Example of Modbus RTU Master MSG Setup

NOTE: The ML1200/1500 communication port must be configured for Modbus RTU Master

before RSLogix500 will present the Modbus commands within the MSG set-up screen.

NOTE: MB Data Address in the Target Device is a word offset into that Modbus RTU Slaves data

file.

NOTE: When the Modbus Command is set to Input Status Read or Input Status Write the

following Data Table address needs to be set at the bit level instead of word level. The Size in

Elements will than follow suite and request the total number of bits versus words.

NOTE: The Target Device MB Data Address on an Input Status data file will be a bit offset

rather than a word offset.

NOTE: If Modbus Command is set to Input Status Read or Write and the following Data Table

address is not configured to the bit level RSLogix500 will report the following error.

NOTE: When configuring Modbus RTU Slave an Expanded button will appear next to the

Modbus Holding Registers data file. The expanded button was added in later FRN and software

to allow consecutive "same type" data files to be linked together to create a larger Modbus

Holding Register data file that is greater than 256 words. The corresponding data files must be

manually created. RSLogix will limit user to 256 for the Holding Registers even though the

Expanded box is checked. Enter in the max value of 256 for the first file.

Troubleshooting

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NOTE: If the Modbus RTU Slave device doesn't respond, the MSG will error out with a time-out

error 0x37. Error code 0x37 is due to cabling or channel configuration problem.

NOTE: If the Modbus RTU Slave device responds with a Modbus error, then the MSG will error

out with a 0x81-0x89 error (see page 21-47 of 1762-RM001E for descriptions). At this point

cabling and Modbus RTU Master channel configuration is correct.

Catalog Number:

1762L24AWA;1762L24BWA;1762L24BXB;1762L40AWA;1762L40BWA;1762L40BXB;1764LRP;1764LSP

DocFullNum: A62452563

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