schneiderModbus Master (Serial and TCP/IP) Driver for Field Device System Integrator Modules

The Field Device System Integrator (FDSI) Modbus Master driver is designed to smoothly integrate single or 

dual-ported Modbus devices using several different Modbus protocols into an I/A Series® system.

FEATURES

Key features of the FDSI Modbus Master driver are:

 Integration of single and/or dual-ported Modbus 

devices into an I/A Series system at the fieldbus 

level

 Exchange of real-time data between Modbus 

devices and the I/A Series system

 Compliance with the globally accepted and 

widely used Modbus communication standard

 Simplex (FBM230 or 232) or Redundant 

(FBM231 or 233) operation of FDSI Fieldbus 

Modules

 Support for three Modbus protocols: Modbus 

RTU and Modbus ASCII protocols for serial 

devices, and Modbus/TCP for Ethernet devices

 Optionally available vendor-specific device 

variations of Modbus drivers 

 FDSI hardware provides a barrier for network 

security issues

 Availability of standard I/A Series plant 

management functions and operator displays to 

Modbus devices

 System Monitor detection of Modbus device 

connectivity

OVERVIEW

The Field Device System Integrator (FDSI) Modbus 

Master driver enables the integration of vendorindependent single or dual-ported Modbus devices 

using one of several Modbus protocols into an 

I/A Series system.

The Modbus Master driver allows the I/A Series 

system to obtain real-time data from attached 

Modbus devices, manipulate the data using state-ofthe-art control algorithms, and write output data to 

the devices. 

The Modbus Master driver is downloaded to the 

appropriate single or redundant FBM, and allows the 

FBM to translate the data sent to the I/A Series 

system from the field device or vice-versa. 

Figure 1 illustrates a typical configuration in which a 

simplex FBM230 is communicating with serial 

Modbus devices. Figure 2 illustrates a typical 

configuration in which an FBM is communicating with 

Ethernet Modbus devices.

HARDWARE

Devices with which the FDSI FBMs can successfully 

communicate are single or dual-ported Modbus 

devices that support any of the three Modbus 

communication protocols. Depending on which 

Modbus devices and methods of communication you 

are using, as well as redundancy requirements, you 

need one or more of the following FDSI FBMs.

FBM230

The FBM230 is equipped with four ports, each of 

which can be configured to use either the RS-232, 

RS-422, or RS-485 physical interface standard. The 

FBM230 supports single-ported devices utilizing 

either the Modbus RTU or Modbus ASCII 

communication protocols, and provides a serial 

interface between the I/A Series system and the 

devices. Refer to PSS 21H-2Z30 B4.

FBM231

The FBM231 is equipped with four ports, each of 

which can be configured to use either the RS-232, 

RS-422, or RS-485 physical interface standard. Two 

FBM231s can be used to implement a redundant 

serial interface between the I/A Series system and 

dual-ported devices utilizing either the Modbus RTU 

or Modbus ASCII communication protocols. Refer to 

PSS 21H-2Z31 B4.

FBM232

The FBM232 supports single-ported devices utilizing 

the Modbus/TCP communication protocol, and 

provides an Ethernet interface between the I/A Series 

system and the devices. Refer to PSS 21H-2Z32 B4.

FBM233

Two FBM233s can be used to implement a 

redundant Ethernet interface between the I/A Series 

system and single or dual-ported devices utilizing the 

Modbus/TCP communication protocol. Refer to 

PSS 21H-2Z33 B4.

MODBUS MASTER DRIVER BENEFITS

Using the Modbus Master driver with FDSI FBMs 

provides the benefits described in the following 

subsections.

Easy Integration of Modbus Devices

The Modbus Master driver allows you to include new 

or existing Modbus-compliant devices in your 

process, regardless of device manufacturer. 

Incorporating an open communication standard like 

Modbus in your system provides a wide range of 

options when selecting the best field devices for your 

process. 

I/A Series System Support

The I/A Series system supports three protocols for 

communication with Modbus devices: Modbus RTU, 

Modbus ASCII, and Modbus/TCP. For FBMs 230 and 

231, either Modbus RTU or Modbus ASCII are 

separately configurable on each port. This support 

accommodates the transfer of data to and from both 

single and dual-ported Modbus devices, via a serial 

or Ethernet connection. 

I/A Series software also provides standard plant 

management functions and operator displays for 

these devices, in addition to startup and 

communication fault detection and display using 

System Monitor.

Flexibility

The Modbus/TCP protocol uses Ethernet and TCP/IP 

technologies. The simplicity and low cost of Ethernet 

hardware combined with the high speeds and 

reliability characterized by Ethernet networks greatly 

improves plant efficiency and provides the flexibility 

required to constantly keep up with changing 

technology. Using an Ethernet network, you can 

provide remote access to process data over the 

network.

Redundancy

A pair of FBM231 or 233 modules combine to 

provide redundancy at the FBM level. In redundant 

configurations, one FBM serves as the Master 

module and the other as the Tracker module. (Both 

are Modbus Masters on their respective networks.) 

A communication link between the two modules 

allows each module to monitor the other's health and 

to share information about the health of the attached 

slave devices. For illustrations of typical 

configurations in which redundant FDSI FBMs 

communicate with serial and Ethernet Modbus 

devices, refer to PSS 21H-2Z31 B4 and 

PSS 21H-2Z32 B4.

Detection of communication issues between the 

FBM and the field are determined by the device 

heartbeat or scan message. 

If a message is missed, up to two retries are 

performed. If the retries fail, the device is marked as 

failed in System Monitor displays.

If a device failure is detected, the Master requests an 

automatic role switch and generates a system alarm. 

Role-switching occurs without a loss of data. 

Alternatively, you can initiate a role switch between 

Master and Tracker modules manually using 

I/A Series System Monitor.

If any one FBM fails, the failure is indicated in System 

Monitor. If a role switch has occurred and you are 

running with a failed FBM, the new Master will 

continue to detect and alarm device failures. 

When the System Monitor alerts you to FBM or 

network connection failures, you can then replace the 

faulty FBM or repair the network connection without 

upsetting field input or output signals on the 

functional side. When a failure occurs, it is 

recommended that you repair or replace the faulty 

component as soon as possible to prevent possible 

loss of data if another failure should occur.

Custom Drivers

Device-specific drivers can be created for unique 

process requirements. For example, a custom driver 

can be created to support Modbus slave devices, to 

perform non-standard data type conversions in 

systems that utilize a combination of data types, or to 

support non-standard Modbus protocol 

implementations. Contact your local Invensys 

Foxboro sales representative for information on 

specific custom drivers that may be available for your 

system.

Diagnostics

System Monitor watches for loss of communication 

between the Modbus field devices and the FDSI 

FBM. FBMs scan their connected devices at a userspecified time interval. If the FBM does not receive a 

response from the device, System Monitor displays 

show that the device has failed. 

The driver can be configured to send heartbeat poll 

messages to devices that support function code 8. 

This enables you to monitor device connection.

OVERVIEW OF MODBUS TECHNOLOGY 

The Modbus Protocol was introduced by GouldModicon® in 1979 as a proprietary message 

structure and format that allowed communication 

and transfer of control data between intelligent field 

devices via RS-232. Modbus/TCP was introduced 

later by Schneider Automation® to support Ethernet 

TCP/IP as an additional data transmission 

technology for the Modbus protocol. 

Today, Modbus has evolved into a globally accepted 

and widely used standard for communication 

between multi-vendor field devices. The Modbus 

protocol specifies the master-slave/query-response 

message structure that controllers can recognize and 

use.

Messaging Operation

Query messages, sent by Modbus masters to 

devices, include a destination address, a task 

request indicated by a “function code”, and any 

supporting data required for the slave to carry out the 

assigned task. The message can be sent directly to a 

particular slave by including its unique address in the 

message. 

After receiving a query message from the master, the 

Modbus device responds with a message. The 

response message specifies whether or not the 

requested action was successfully performed and 

confirms that the correct Modbus device responded 

to the requested action. The response message also 

returns any data requested by the master.

Query and response messages both contain error 

checking mechanisms that allow devices to confirm 

the integrity of the query message and allow the 

master to confirm the integrity of the response. If the 

error check is not correct, the associated query or 

response message is ignored and the transaction is 

retried. 

Modbus Master Device Driver Operation

Modbus “function codes” specify the types of actions 

the Modbus devices must perform. The following 

Modbus function codes are supported by the FDSI 

Modbus Master Driver:

* Data diagnostic code only.

You do not need to specifically configure Modbus 

transactions, which makes driver configuration very 

easy. By defining the necessary device register 

addresses in DCI blocks, the Modbus driver 

automatically determines the necessary transactions.

After the appropriate DCI blocks and ECBs are 

configured and device communication is enabled, the 

FDSI FBM starts sending “scan” messages to each 

Modbus device. The FBM waits for a response from 

the device, processes the data contained in the 

response, and stores the data in its database for 

incorporation into the I/A Series control system. Data 

can also be written out to the individual devices from 

the I/A Series network.

Installation and Download

Installation of the driver does not require shutting 

down the I/A Series software or rebooting the 

I/A Series workstation. Furthermore, any updated 

driver can be downloaded to the FBM or FBM pair 

without disrupting the rest of the I/A Series system.

CONFIGURATION

To configure the FDSI Modbus Master driver, you 

must use the FDSI Configurator software, which can 

be installed on a workstation running the 

Windows XP® operating system. The FDSI 

Configurator is required for configuring FBM serial 

and Ethernet port properties and communication 

settings. 

Device Configuration

Device configuration for the Modbus Master driver is 

performed entirely in DCI blocks. There are no scan 

ECBs or device configuration files required or used 

by the driver. Once you have configured DCI blocks, 

the driver builds the required Modbus scan 

transactions to transfer data. 

SPECIFICATIONS

Protocols

The Modbus Master driver offers support for the 

following protocols:

 Modbus RTU protocol (for serial devices)

 Modbus ASCII protocol (for serial devices)

 Modbus/TCP protocol (for Ethernet devices)

For additional information, refer to the following 

Modbus protocol documentation:

 Modicon Modbus Protocol Reference Guide

(Modbus RTU and Modbus ASCII protocols) 

PI-MBUS–300 Rev. J, MODICON, Inc. Industrial 

Automation Systems, June 1996 

 Open Modbus/TCP Specification (Modbus/TCP 

protocol) Release 1.0, Schneider Electric, 

March 1999.

Register Address Support

The Modbus Master driver supports both five digit 

and six digit register addresses.

Data Type and Format

The Modbus Master driver accepts data from 

devices in big endian or little endian format; data 

format is configurable. 

Number of Devices

Up to 64 devices per FDSI FBM maximum. The 

number of actual devices is performance and 

configuration dependent.

Number of Devices per FBM

 RS-232: 1 device per port

 RS-422: 1 device per port

 RS-485: 16 devices per port

 Ethernet: multiple devices per FBM or FBM pair. 

(The number of actual devices is performance 

dependent.)

Number of Points

Up to 2000 I/O points can be handled in an FDSI 

FBM. The standard Modbus Master driver uses a 

default 500 millisecond scan cycle, but can be 

configured to other rates in 100 ms increments to as 

fast as 100 ms. As a result, the practical limit on the 

number of points that can be supported depends on 

the update rate you require. The throughput rate is 

determined by:

 Network limitations.

 Efficiency of the database (accessing scattered 

registers in a Modbus device requires more 

Modbus messages than accessing contiguous 

register numbers).

 Response time of the connected devices.

Control Block Support

Distributed Control Interface (DCI) blocks address 

and read/write data from/to the addressed slave 

device. The Modbus driver offers control block 

support for the following ECBs and standard DCI 

block types:

Table 1. ECBs Supported by the Modbus 

Master Driver

ECB200 Parent ECB, representing the FBM230 

or FBM232

ECB202 Parent ECB, representing the FBM231 

or FBM233

ECB201 Child ECB, representing a device

Table 2. DCI Blocks Supported by the Modbus 

Master Driver

BIN Binary Input block

BINR Redundant Binary Input block

BOUT Binary Output block

IIN Integer Input block

IOUT Integer Output block

PAKIN Packed Input block

PAKOUT Packed Output block

PLSOUT Pulse Output block

RIN Real Input DCI block

RINR Redundant Real Input DCI block

ROUT Real Output DCI block