Modbus Serial Channel Tag

Introduction

The Modbus Serial Channel Tag provides a serial SCADA communication channel to Modbus Devices.

Configuration Properties

The Modbus serial channel supports the properties of the Modbus Channel Tag plus these properties:

Property	Type	Version	Description
Port	WSTRING	1.0	The name of the communications port. For example 'COM1:'
Mode	WSTRING	1.0	The serial port communication speed, data format and handshake mode. Click the Browse button to edit the serial port mode using the Serial Port Mode Editor. You can also type in the serial port mode string directly. See Serial Port Mode String for how this property is formatted.
Protocol	USINT	1.0	The communication protocol: Modbus RTU Modbus ASCII
TransmitDelay	UINT	3.34	The number of characters to wait before sending the next Modbus RTU message. Only applies if Protocol is set to Modbus RTU.

TransmitDelay property

The Modbus RTU specification requires each message starts with a silent interval of at least 3.5 characters.

The TransmitDelay property sets the number of characters to wait before Fernhill SCADA sends the next message. The Fernhill SCADA Modbus driver waits for this interval before sending the next message.

The actual interval is calculated from the following parameters:

The baud rate, for example 9600.
The number of bits used to transmit each character, for example 11.
The value of the TransmitDelay property.

If the calculated value is less than 5ms, the value is rounded up to 5ms.

Not all Modbus RTU devices require a silent interval. Providing the silent interval maximizes interoperability with Modbus devices that require it.

The TransmitDelay is not applied if Protocol is set to Modbus ASCII.

Tag Commands

The Modbus Serial Channel Tag supports the tag commands of the Modbus Channel Tag.

Example Configuration

This is an example of channel configuration suitable for Modbus RTU:

Property	Value
ReplyTimeout	T#5s
Port	COM1
Mode	Baud=9600 Parity=N Data=8 Stop=1
Protocol	Modbus RTU

Notes:

The serial port is COM1.
The Modbus protocol is RTU.
The bit rate is 9600 Baud.
The serial character framing is 8 data bits, no parity and 1 stop bit.
The reply timeout is 5 seconds.

Relationships to Other Tag Types

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

Further Information

Modbus Protocol

To learn more about the Modbus Protocol.

Active Channel Tag

To learn about the properties common to all active channels.

Modbus TCP Channel Tag

To learn about Modbus TCP Channel Tags.

Modbus Device Tag

To learn about Modbus Device Tags that can be associated with Modbus serial channels.

Alarm Conditions

To learn about abnormal conditions in SCADA applications.

Serial Port Mode Editor

To learn how to set the communications speed, data format, and handshake mode of the serial port.

Glossary

For the meaning of terms used in Fernhill SCADA.