White Paper. Technical Capabilities of the DF1 Half-Duplex Protocol

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T ech n ica l C ap ab ilities o f th e D F 1

H alf-D u p lex P rotocol

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Introduction

network, systems must offer customers reliability and flexibility.

Allen-Bradley’s DF1 with half-duplex is just one of many protocol solutions for networking two or more intelligent devices together. The protocol is based on the RS-232C compatible serial interface, which defines the connector pinouts, cabling, and signal levels. This interface is the electrical and physical connection. This document only describes the protocol. No information concerning the RS-232 hardware will be discussed.

A primary application for both protocols is SCADA (Supervisory Control and Data Acquisition). SCADA networks allow users to control and monitor various remote functions and processes. In each of the following network drawings, the master polls each slave in a predefined order. Through this process, the master can oversee the slaves’ control, maintain the process, and react to changes at the remote sites. These networks can support many media links such as radio/ microwave modems, dialup modems, and power line modems. Historically, SCADA users have utilized proprietary costly equipment to provide control and intelligence. With the introduction of micro PLCs that support these protocols, a low-cost intelligent off-the-shelf solution now exists.

DF1 Protocol

DF1 is an open and freely available protocol developed by Allen-Bradley. In use for over 15 years, DF1 is used for two-way data transmission, point-to-point, and multi-drop networks in local and long-distance applications.

The DF1 protocol can be configured for full or half-duplex operation. DF1 full duplex is used over a point-to-point link (one device connected to another). This allows for simultaneous bi-directional communications and produces higher throughput for high performance applications. Half-duplex is for master/slave multi-drop communications. This allows bi-directional communications in one direction at a time.

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DF1 and Half-duplex

DF1 half-duplex is used for drop applications such as SCADA. The drop link utilizes one master and one or more slaves as shown above. The multi-drop link is a network with all nodes/stations interfaced through compatible devices (modems, line drivers, etc.). There can be up to 255 devices connected on each network. When using half-duplex networks, one node-station is always designated as the master station. The master controls all communications activity on the network; other nodes/stations are slaves and must wait for permission from the master before communicating.

On a half-duplex network, each slave has a unique node/station address from 0 to 254. The master communicates with each station by “addressing” each message to the appropriate station address. This allows the master to selectively

communicate with a specific node/station. If the master needs to send data to more than one station at the same time, a “broadcast” capability is built into DF1. If the master sends a message with a node address of 255, all connected devices “detect” and accept the message.

Slave-to-slave

Messaging

One of the unique features of DF1 is its ability to let slave devices send messages directly to other slaves devices. This capability is called “slave-to-slave

messaging” and allows a programming terminal to reside on the network as the slave. The programming terminal can both download/upload and monitor another slave. The DF1 master allows this type of messaging because it is built into the

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DF1 protocol, allowing a slave to initiate a message. When the master station polls the slave station, it detects a pending message. When the master reads the message, it forwards the message on to the appropriate station. This is done without any programming in the DF1 master.

DF1 Message Types

Master Command Message: This message is used when the master needs to

address only one slave on the network, If the master at address 1 needs to send a message to a slave at address 2, the master places a value of 2 in the slave address and sends the message. In the master message block the command field in the data field tells the slave at address 2 what kind of action to perform. The slave responds back to the master acknowledging it received the message. When the master needs to broadcast a single message to all slaves on the network, a 255 is placed in the slave address field.

This is an example of a DF1 message structure.

Start of Message Slave Address: 02 Data Separator Data End of Message CRC Error Check Acknowledge Destination 02 Source 01 Command or

Reply Status Trans # Data

Start of

Message Data

End of

Message Error Check Acknowledge

Destination 01

Source 02

Command or

Master command message Slave Response

Master Response Slave command message

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The master polls a slave based on how the master is configured. If the slave has a message pending (initiated by the event or change-of-state), the slave sends it when the master polls the slave.

This is an example of master poll sequence.

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Summary

DF1 is an open and freely available network protocol. It can be configured for full or half-duplex operation. DF1 full-duplex is used in point-to-point links with one device connected to another. Half-duplex is for master/slave multi-drop networks in local and long-distance applications. These multi-drop networks are referred to SCADA networks. When configured for DF1 half-duplex, there can be up to 255 devices connected on one multi-drop network. The devices on a multi-drop network are interfaced through compatible communication hardware (modems, line drivers, etc.).

A unique feature of DF1 half-duplex is that is offers slave-to-slave messaging. This makes it easier to send data from a slave to another slave. DF1 half-duplex also allows a programming station to be connected as a slave device and allows the user to download, upload, and monitor programs in another slave on the network from the programming station. These features do not require any special programming in the master. These features are inherent with Rockwell

Automation products that support DF1 half-duplex master/slave protocol. This includes products like the MicroLogix 1000, SLC 5/03, SLC 5/04, SLC 5/05, and the PLC-5 series of PLCs.

The selection of a SCADA network protocol is a significant decision and is not

Start Poll Command

Station

Number Error Check Acknowledge

No message Acknowledge

Start of

Message Data

End of

Message Error Check Acknowledge

Destination Source Command or

Slave Response

Master Response

Master Response Master polling packet (4 bytes)

Slave without data (2 bytes)

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Publication 1761-WP003A-EN-E - March 2001

Definitions

ASCII - American Standard Code for Information Interchange.

DF1 - DF1 protocol is based on ANSIx3.28 specification. It is an open protocol

used for two-way data transmission.

Multi-drop - Multiple connections of more than two (2) stations. Point-to-Point - A single connection between two (2) stations. Protocol - Defines the format or structure of data exchanged over a

communications link between two (2) devices/stations.

Report by Exception: A method of initiating communications, typically when an

event or action is detected.

RTU: Remote Terminal Unit.

SCADA - Supervisory Control and Data Acquisition.

Topology: The way the network is physically structured. Example: a ring, bus or