Modbus Device Tag

Introduction

The Modbus Device Tag represents a logical link to a device that supports the Modbus protocol.

Configuration Properties

The Modbus Device Tag supports the configuration properties of the PLC Scanned Device Tag, plus these properties:

Property	Type	Version	Description
Channel	Reference	1.0	The Modbus Communication Channel to the device.
Unit	USINT	1.0	The Modbus unit address of the device.
DataAddressFormat	USINT	3.86	Defines the format of data address properties. The options are: Standard Modbus Enron Modbus DirectLogic See DataAddressFormat Property for more information.
WordOrder	USINT	1.1	For multi-word tags, the order of words. 0 = High word first. 1 = Low word first.
HasCommand05	BOOL	3.36	Whether the PLC Device supports Modbus command 05 (force single coil).
HasCommand06	BOOL	3.36	Whether the PLC Device supports Modbus command 06 (preset single register).
HasCommand15	BOOL	3.36	Whether the PLC Device supports Modbus command 15 (force multiple coils).
HasCommand16	BOOL	3.36	Whether the PLC Device supports Modbus command 16 (preset multiple registers).
HasCommand22	BOOL	3.35	Whether the PLC Device supports Modbus command 22 (mask write holding register).
PollMethod	USINT	3.35	How to test whether the device is online. 0 = Automatic, 1 = Read Register.
PollRegisterAddress	STRING	3.35	When PollMethod is set to Read Register, the register address to read.
PollRegisterCount	UINT	3.35	When PollMethod is set to Read Register, the number of registers to read.

DataAddressFormat Property

In a Modbus Device, a Data Address is the location of an item of data in a Modbus Device. The DataAddressFormat property defines the format of a Data Address.

These Data Address Formats are supported:

DataAddressFormat	Address Format
Standard Modbus	The address uses the standard Modbus convention. For more information see Standard Modbus Data Address.
Enron Modbus	The address uses the Enron Modbus convention. For more information see Enron Modbus Data Address.
DirectLogic	The address uses the DirectLogic convention. For more information see DirectLogic Data Address.

HasCommand05 and HasCommand15 Properties

For Standard Modbus, the HasCommand05 and HasCommand15 properties tell the driver the available commands to write output coils.

Most Modbus Devices support both command 05 (write single coil) and command 15 (write multiple coils). If a Modbus Device does not support command 05, set HasCommand05 to False. If a Modbus Device does not support command 15, set HasCommand15 to False.

This table shows how the driver writes output coils depending on the configuration of HasCommand05 and HasCommand15:

HasCommand05	HasCommand15	Description
True	True	Single bit writes use command 05. Multiple bit writes use command 15.
False	True	Single and multiple bit writes use command 15.
True	False	Single bit writes use command 05. Multiple bit writes result in an error.
False	False	Single and multiple word writes result in an error.

HasCommand06 and HasCommand16 Properties

For Standard Modbus, the HasCommand06 and HasCommand16 properties tell the driver the available commands to write holding registers.

Most Modbus Devices support both command 06 (preset single register) and command 16 (preset multiple registers). If a Modbus Device does not support command 06, set HasCommand06 to False. If a Modbus Device does not support command 16, set HasCommand16 to False.

This table shows how the driver writes holding registers depending on the configuration of HasCommand06 and HasCommand16:

HasCommand06	HasCommand16	Description
True	True	Single word writes use command 06. Multiple word writes use command 16.
False	True	Single and multiple word writes use command 16.
True	False	Single word writes use command 06. Multiple word writes result in an error.
False	False	Single and multiple word writes result in an error.

HasCommand22 Property

For Standard Modbus, the HasCommand22 configuration property tells the driver whether the Modbus Device supports Modbus command 22. Modbus command 22 is a masked write operation to a holding register. It allows the Modbus master to set or clear bits in a holding register in one operation. Not all Modbus Devices support this command.

The value of this property affects how the Modbus driver writes bits to holding registers:

HasCommand22	Driver Action
True	Use command 22 to set or clear the bit.
False	Use a three step process to set or clear the bit: Use command 3 to read the holding register. Set or clear the required bit. Use command 6 to write the holding register back.

If the Modbus device supports Modbus Command 22, set HasCommand22 to True.

When HasCommand22 is False, avoid writes from Fernhill SCADA to holding registers that are also updated by the PLC program. The following conditions could result in unexpected changes to holding register values:

HasCommand22 is False, and
Fernhill SCADA writes a bit value to a holding register, and
The PLC program is also updating the holding register.

PollMethod Property

The Modbus driver sends a poll command the PLC Device at a regular interval to check if the PLC Device is online. Provided the PLC Device sends a response to the poll command, the PLC Device status will be online. The PLC Device status will be online even if the PLC Device replies with an exception response.

The PollMethod property tells the driver what Modbus command to use as a PLC Device Poll. These options are supported:

PollMethod	Description
Automatic	The driver tries three different poll methods in turn: Diagnostics command 08 with sub-function 00 (Return Query Data), or Read Holding Register 40001 Read Output Coil 00001 Once a working poll method has been found, the driver will continue to poll the device with the working poll method.
Read Register	The driver reads the register defined by the PollRegisterAddress property. The PollRegisterCount property tells the driver how many registers to read.

The PollMethod is an advanced configuration option. In most applications PollMethod should be set to the default option: Automatic.

A few PLC devices do not implement Modbus correctly. For example:

Some devices provide no response to unknown commands. The correct Modbus behavior is to provide an exception response.
A few devices place restrictions on the address and length fields of a read request. For example, a device might only support reading three registers from specific locations.

Setting PollMethod to "Read Register" and configuring PollRegisterAddress and PollRegisterCount appropriately may help establish communications with devices that do not implement Modbus correctly.

Tag Commands

The Modbus Device Tag supports the tag commands of the PLC Scanned Device Tag, plus these properties:

Tag Command	Version	Description
DirectWriteWord	2.3	Directly writes one or more words to an address in the PLC.
DirectWriteBit	2.3	Directly writes a single bit to an address in the PLC.

Example Configuration

This is an example of a Modbus device at unit address 1, communicating via a channel called ".Remote Plant":

Property	Value
Channel	.Remote Plant
Unit	1

Relationships to Other Tag Types

The Modbus Device Tag builds on the features of these tag types:

Further Information

PLC Device Tag

For properties common to all types of device.

Modbus Serial Communication Channel

For more information on serial communications.

Modbus TCP Communication Channel

For more information on setting up a TCP/IP communication channel for Modbus.

Modbus Register Blocks

For more information on the Modbus Register Block Tags that can be associated with Modbus Devices.

Alarm Conditions

To learn about abnormal conditions in SCADA applications.

Glossary

For the meaning of terms used in Fernhill SCADA.