Modbus_RTU.pdf

Modbus RTU Master Communications

This document describes the operation of Modbus ® RTU Master from the user interface

point of view. Use this information as a supplement to the Serial Communications User’s

Manual (GFK-0582). This document contains the following information:

Supported Products

Mode of Operation

COMMREQs

Time Constraints

Serial Connections

Multidrop Connections

Drivers and Receivers

Cable

Grounding

Connector Wiring

Four-Wire

Two-Wire

Point-to-Point

COMMREQ Function Block Format

COMMREQ Function Block Output

COMMREQ Command Data Block

Modbus RTU Master Status Word Format

Queue Limitations for Modbus RTU Master COMMREQs

Local Data

Modbus RTU Master Diagnostic Status Words

Modbus RTU Master Commands

Clear RTU Master Diagnostic Status Words: 08000 (1F40)

Read RTU Master Diagnostic Status Words: 08001 (1F41)

Send RTU Read/Force/Preset Query: 08002 (1F42)

Send RTU Diagnostic Query: 08003 (1F43)

Error Codes

Example Application

® Modbus is a registered trademark of Schneider Electric.

GFK-2220B

Overview

Standards

COMMREQ Function Block Parameters

Initialize RTU Master Port: 65520 (FFF0)

Overview

Modbus Serial Line protocol is an open standard for data communications between PLCs

and related devices. The Modbus Serial Line standard provides for communication using

either printable characters exclusively (Modbus ASCII), or binary data (Modbus RTU).

This document describes Modbus RTU Master communications on GE Fanuc PLC CPUs.

Supported Products

Modbus RTU Master communications is currently available on the Series 90 -30

IC693CPU363 and VersaMax ® modular CPU models IC200CPU001, IC200CPU002,

IC200CPU005 and IC200CPUE05.

Mode of Operation

A Modbus RTU master device (the client) sends query messages to one or more slave

devices (the servers) on a serial network. Queries may contain data, requests for data or

status, or commands.

Each slave on the network has a unique device address. Any query may be addressed

either to a specific slave device or to a special broadcast address. Queries addressed to the

broadcast address are called broadcast queries . Queries that require a response may not

be addressed to the broadcast address.

A slave that receives a well-formed, non-broadcast query must send a response message to

the master. The query/response transaction completes when the master receives a well-

formed response.

Slaves do not respond to broadcast queries. After sending a broadcast query, the master

must wait a specified time before completing the transaction and sending the next query.

Some broadcast queries contain commands that require the slaves to take specified

actions.

COMMREQs

The application program running in the PLC CPU controls the timing and content of each

query by sending a COMMREQ message. The COMMREQ must be addressed to the

CPU serial port that is connected to the Modbus RTU serial network. COMMREQ data

specifies the content of the query. When the query/response transaction completes, a

COMMREQ status value indicates the success or failure of the transaction. See the

COMMREQ Function Block Format section of this document and the sections that follow

it for details.

Time Constraints

The Modbus over Serial Line Specification and Implementation guide V1.0 1 contains

several important timing requirements.

Serial Connections

Series 90 and VersaMax are trademarks of GE Fanuc Automation.

1 The current Modbus RTU specification is available online in the documents Modbus Application

Protocol Specification V1.1 and Modbus over Serial Line Specification and Implementation guide V1.0

at http://www.modbus.org/. Follow the Modbus Standard Library link to find them .

Modbus RTU Master Communications - December 2003

GFK-2220B

A Modbus RTU network has one master device and one or more (up to 247) slave devices.

A serial network interconnects all these devices. If there is only one slave, a point-to-

point connection is used. A multidrop connection is needed for two or more slaves.

Standards

Virtually all PLC serial communications ports (including all serial ports on GE Fanuc PLC

CPUs) support one (or two or three in some cases) of three physical layer standards for

asynchronous serial communications. The current revisions of all three may be purchased

from the Telecommunications Industries Association at

http://www.tiaonline.org/standards/ .

EIA/TIA-232-F: Interface Between Data Terminal Equipment and Data Circuit-Terminating

Equipment Employing Serial Binary Data Interchange (ANSI/TIA/EIA-232-F-1997)

This standard is commonly referred to as “RS-232” or “RS-232C” because the definitive

earlier revision was titled “RS-232-C”. RS-232 ports transmit and receive data and

control signals on unbalanced circuits. That is, one Signal Common (or Signal Ground)

wire serves as the return path for all the data and control circuits.

RS-232 ports are suitable for point-to-point connections up to about 25 meters in length,

but not for longer lines or multidrop connections. The specification recommends limiting

the data rate to (in effect) 19,200 bits per second (bps) or less, but rates up 115,200 bps are

frequently used with short cables (typically about 2 meters).

EIA/TIA-422-B: Electrical Characteristics of Balanced Voltage Digital Interface Circuits

(ANSI/TIA/EIA-422-B-94, revised 2000)

This standard is usually called “RS-422” because the initial revision had that title. It

specifies twisted-pair cabling and a balanced line driver and receiver for each circuit. RS-

442 supports higher data rates and longer distances than RS-232. A 100-ohm nominal

impedance is recommended for twisted pair circuits in cables, and 100-ohm terminating

resistors are recommended for the receiving end of each circuit.

Some RS-422 ports support multidrop (multipoint) operation. However, this capability is

not guaranteed by the standard. Use caution when attempting to use an RS-422 device on

a multidrop network.

EIA/TIA-485-A: Electrical Characteristics of Generators and Receivers for Use in Balanced

Digital Multipoint Systems (ANSI/TIA/EIA-485-A-98)

The original version of this standard was titled “RS-485”; it is frequently referred to by that

name. This standard has effectively replaced RS-422 because it adds guaranteed multidrop

(multipoint) capability. Line drivers in the data circuits are required to switch to a high-

impedance state (“tristate” themselves) except when transmitting, and the control and status

circuits are rarely connected through the cable in multidrop applications. Consequently,

multiple data line drivers can be connected in parallel to each data circuit. The port

firmware guarantees only one port at a time will attempt to transmit on each circuit.

RS-485 uses 120-ohm cable and terminating resistors. Because transmitters are not

always connected to the line, terminating resistors must be used at both ends of each

circuit.

Note that some RS-485 devices may require pull-up and pull-down resistors to polarize

(bias) receive-data circuits to the mark state when all transmitters are in the high-

impedance state. GE Fanuc RS-485 ports do not require pull-up and pull-down resistors.

GFK-2220B

Multidrop Connections

Four-Wire (4-Wire)

In this network configuration, the Send Data (SD) pair of the RTU master device is

connected to the Receive Data (RD) pairs of all the slaves, and the SD pairs of all the

slaves are connected to the RD pair of the master. The slave devices must all use RS-485-

compatible serial ports so that their transmitters are disabled except when transmitting.

Although some RS-442 devices disable outputs when not transmitting, the RS-442

specification does not require it. The master may use either an RS-422 or RS-485 port

because it is the only transmitter on that pair. Serial ports on all devices should be

configured for Flow Control NONE.

Both signal pairs must be terminated at both ends by appropriate resistors. Each pair is

terminated at one end by the resistive terminator (RT pin) build into the end port. The

other end of each pair is terminated by a 120 ohm, ¼ watt resistor in the connector shell.

If the end device lacks a built-in terminator, a second 120-ohm, ¼ watt resistor must be

wired across the RD signal pair inside the connector shell.

Four-Wire Connection without Repeaters

Notes:

1. Connectors on the cable ends have the

Resistive Termination (RT) pin connected.

All others have RT unconnected.

SHLD

N/C

+5VDC

RTS(A)

CTS(B’)

SD(A)

RD(A’)

SHLD

N/C

+5VDC

RTS(A)

2. Terminate the SD signal pair in both end

connectors with a 120 ohm, ¼ watt

resistor in the connector shell.

CTS(B’)

RD(A’)

3. One connector is shown between the end

devices. Additional connectors may be

added up to a total of 31.

4. Attach all connectors to 15-pin RS-485

ports on GE Fanuc CPU modules. The

Modbus RTU master device must be

attached at the left-hand connector. All

other devices are slaves.

5. Jumper connections between CTS and

RTS are optional. No GE Fanuc 15-pin

RS-485 ports that support Modbus RTU

RD(B’)

SD(B)

SD(A)

SD(B)

RTS(B)

RD(B’)

RTS(B)

CTS(A’)

120

SLAVE CONNECTORS

MODBUS RTU MASTER

(END CONNECTOR)

SHLD

N/C

currently require them.

6. There are ground loops through the SHLD

connections and frame ground connections

of the master and slave devices. Large

ground loop currents can cause data

errors. See the accompanying text for a

discussion of this issue.

7. Vary the connector wiring as needed to

accommodate third-party RTU slave

devices.

+5VDC

RTS(A)

CTS(B’)

RD(A’)

RD(B’)

SD(A)

SD(B)

RTS(B)

120

CTS(A’)

END CONNECTOR

Any high-quality shielded twisted-pair cable with two pairs is suitable for short cable runs

(up to about 15 meters). Longer runs require a cable with a nominal impedance of 120

ohms. Use a cable designed for RS-485 transmission such as Belden 2 9842 or equivalent.

2 Belden is a trademark of Belden Technologies Inc .

Modbus RTU Master Communications - December 2003

GFK-2220B

N/C

Grounding and Ground Loops

Proper grounding of the cable shield requires careful planning of the network and its

power wiring. To avoid data errors from intermittent electrical noise, it is vital to ground

the cable shield to the SHLD (shield) pin of every device on the network. Unfortunately,

this introduces at least N-1 ground loops, where N is the number of devices on the

network. Each ground loop path comprises the shield and drain wire on the cable segment

between two devices and a ground return path. The return paths start at the frame ground

point of one device, pass through its ground conductor to the common ground, and then

pass through the ground conductor of the other device to its frame ground point.

Ground loop currents must be kept within acceptable limits by careful grounding.

Otherwise, common-mode noise induced on the data pair by the ground loop currents can

cause data errors.

When designing ground wiring, consider these requirements:

1. There must be one common ground point in the system with an extremely low

impedance path to earth.

2. The conductor from the frame ground point of each device to the common ground

must have extremely low impedance.

3. The recommended frame ground wire sizes, lengths and proper wiring practices must

be observed in designing the connections between frame ground points and the

common ground.

4. The data cable and ground wire routing must be physically isolated from other wiring

that could couple noise onto the data cable or ground wiring.

5. If disconnecting the cable shield from the SHLD pin on any device reduces data

errors, the network has a ground loop issue. Some older versions of GE Fanuc

manuals recommend connecting cable shields at one end only to eliminate ground

loop currents. This is no longer a recommended practice. When a shield is grounded

at only one end, the network’s susceptibility to intermittent data errors from

electromagnetic interference (EMI) is increased significantly. These errors may not

be immediately apparent and often result in substantial post-installation costs for

diagnosis and remediation.

If data errors caused by ground loops cannot be avoided (for example, because the cable

run is too long for all devices to use a common ground point), add one or more optically

isolated RS-485 repeaters to the network. Partition the network into segments so that each

segment has a common ground. Isolate the segments with repeaters such as the BLACK

BOX model IC158A: http://www.blackbox.com/ . See the repeater manufacturer’s data

sheet for details. The figure on the previous page shows a typical 4-wire network without

repeaters.

BLACK BOX is a registered trademark of Black Box Corporation .

GFK-2220B

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