Chapter 11

Introduction To ControlNet

Networks Popularity

• Stand-alone PLCs fading fast• Older networks being upgraded

– Faster more efficient networks– New networks offer deterministic and

repeatable data transfer

Network Advantages for Maintenance Individuals

• PLCs connected on network– Access any PLC from a single computer

anywhere on the network– Upload, download, on-line editing across

network to any device on network from a central location

ControlNet

• Open network managed by ControlNet International• Use for real-time data transfer of time-critical and

non-time-critical data between processors or I/O on same link

• Data transferred at a fixed rate of 5 million bits per second

• ControlNet basically a combination of Allen-Bradley’s Data Highway Plus and Remote I/O

ControlNet Nodes

• Up to 99 nodes• No node 0• Actual number of nodes determined by how

efficiently the network bandwidth is set up• Network set up using RSNetWorx for

ControlNet software

Nodes on ControlNet

• SLC 500, ControlLogix, PLC 5 processors• Third-party field devices• Operator interface• Variable frequency drives

ControlNet Applications

• ControlLogix processor, SLC 500, or PLC 5 processor scheduled data exchange

• Local PLC connection to remote chassis for high-speed remote I/O connectivity

Interlocking or Synchronization of Multiple Nodes

• Synchronized starting of variable frequency drives on ControlNet

• Interlocking multiple processors

Network Bridging

• Connect two Data Highway Plus networks• Connect multiple DeviceNet networks

Trunk Line – Drop Line

Nodenumber

Termination resistor

Control Net Tap

Trunk line

Drop line

Termination resistor

PLC 5 as node 7

SLC 500as node 8Variable

frequency drive as node 4

ComputerInterface cards

Operatorinterface

ControlLogix PLCs

ControlNet Taps

Straight T Straight Y Right Angle Right Angle Y

Drop line length is fixed at 1 meter (39.5 inches).

SLC 500 ControlNet Interface

• 1747-SCNR• SLC 500 modular

PLCs• Scheduled and

unscheduled messaging

ControlLogix ControlNet Interface

• 1756-CNB– Channel A only

• 1756-CNBR – Channel A and B for

redundant media• Module node address

set with side switches• Duplicate node

addresses not allowed• NAP for computer

connectivity

FlexLogix ControlNet Interface

FlexLogix processor

Two communication card slots

Two ControlNet interface cardswith redundancy

NAP

Set node address here

Redundant Media

Tap

Trunkline

PLC 5node

Personal computer with ControlNet interface

Redundant cables

Dropline

ControlLogix

Personal Computer Interface

Personal computer with ControlNet interface card like 1784-KTCX15

Redundant trunk line

KTCX15 ControlNet Interface

Status indicatorsNetwork access port

Channel A

Channel B

Floppy with card driver

KTCX15 Interface to PLC 5PLC 5 ControlNet processor

NAP

Channel A

Channel B

Redundant trunk line

Redundant trunk line

Personal computer with ControlNet interface

NAP connection

Personal Computer to NAP

ControlNet Cabling

ControlNet Segment

Cabling Terms

• Segment• Trunk line cable section• Termination resistor• Link

Segment

• Comprised of a number of sections of trunk cable separated by taps

• Maximum segment length 1,000 meters or 3,280 feet

• Maximum 48 nodes per segment• Segment length determined by number of

nodes

Taps

• Taps are required.• There is no minimum cable length between

taps.• Taps can be directly connected together.

Trunk Line Cable Section

• Trunk line cable section connects one tap to another.

• Taps are required.• Standard light industrial quad shielded RG-6.• Special use cables are available.• Fiber optic cables are available.

Termination Resistor

• One termination resistor is required on the end of every segment.

ControlNet Segment

Termination resistorTermination

resistor39.5 inches

Segment

Trunk line Drop

lineTap

Segment Calculation

• Formula to calculate segment length

1,000 meters – [16.3 meters ( number of taps – 2)]

Calculation Example:

• Calculate maximum segment length using standard light industrial RG-6 coax requiring 22 taps.

1,000 meters – [16.3 meters ( 22 - 2)]

1,000 meters – [16.3 meters ( 20 )]

1,000 meters – 326 meters

Maximum segment length = 674 meters

ControlNet Link

• If more than 48 nodes are required, an additional segment is required.

• Repeater connects segments.• Two segments connected by a repeater is a

link.

Three Segments Connected by a Repeater to Create a Star

Four Segments Connected by Repeaters to Create a Ring

ControlNet Repeaters

• Required if additional nodes are required after either maximum number of nodes or cable length reached

• Two modules required to build a repeater• Many copper and fiber repeaters to select

from depending on application

Building a Repeater Example

• The two repeater modules can be DIN rail-mounted as a pair to build a repeater.– 1 - 1786-RPA (repeater adaptor module)– 1 - 1786-RPCD (dual copper repeater)

Repeater Adapter Module

Dual Copper Repeater Module

Repeaters

ControlNet PLC 5

Add Flex- I/O blocks to this communicationfor remote I/O points

Example of Fiber Repeater

• Fiber repeaters available as short, medium, long, and very long haul

• Up to 18.5 miles ControlNet network using proper fiber repeaters

• Right-hand module in previous slide

RSNetWorx Software

RSNetWorx

• From Rockwell Software• Required to configure and schedule a

ControlNet network

RSNetWorx For ControlNet

Graphic view of network

Go on-line with network

RSNetWorx for ControlNet

Enable editing

Manual networkconfiguration

Networkbandwidth utilization

Trunkline

Nodenumber

How Critical is this Data?

• Separate data into two categories.– Is this information time critical?– Can this information be transferred on a non-

time critical basis?

What is Real-Time for This Application?

• How soon do you really need the information?

• Networks do not have unlimited bandwidth.– Cannot have everything instantly

Realistic Data Flow (1 of 2)

• Assume you had a tank that takes four hours to fill.– Why would you need a tank level every 10

milliseconds?– What is realistic?– Would every few seconds be acceptable?

Realistic Data Flow (2 of 2)

• Assume you had a tank of water that takes two hours to heat.– Why would you need a tank temperature

every 10 milliseconds?– What is realistic?– Would every few seconds be acceptable?

Scheduled / Unscheduled

• Time critical data is scheduled data.– Requested packet interval (RPI) set up in

RSNetWorx• Non-time critical data is unscheduled data.

– Message instruction programmed on PLC ladder rung

• Trigger to transfer only when needed

RPI

• Requested packet interval• Scheduled network service

– The requested interval time-critical data will flow

– ControlNet will meet or beat the RPI if network installed and configured properly

Inefficient Network

• Improper installation– Follow installation manual

• Improper network modification– Follow installation manual

• Poor design– Follow installation manual

• Overdriving network– Unrealistic data flow expectations

ControlNet Bandwidth

• Three pieces to bandwidth– Scheduled traffic– Unscheduled traffic– Maintenance or guard band

Scheduled TrafficUnscheduled Traffic

NetworkMaintenance

Network Update Time

Network Update

• One cycle of the network• Called NUT time

– Set up on RSNetWorx• Data cannot transfer faster than the NUT

Scheduled Maximum Node

• SMAX• This is the highest node number that will be

allowed to send scheduled data.• Any node address above SMAX that has

scheduled data to transfer will not be allowed to transmit.

Unscheduled Maximum Node

• UMAX• This is the highest node number that will be

allowed to send unscheduled data.• Any node address above UMAX that has

scheduled data to transfer will not be allowed to transmit.

Node Can Send Both

• A node can send scheduled as well as unscheduled data/

• The node number must be within SMAX.

Slot Time

• Slot time is time the network spends waiting for a node to respond when the node address is either not used or the node is not responding.

• Unused node addresses should be kept to a minimum for network efficiency.

NUT

SMAX

UMAX

Media and channel information

RSNetWorx Parameters

Set-up Example

• 10 nodes scheduled traffic• 2 spare nodes future scheduled traffic• 14 unscheduled nodes• 3 nodes for NAP connectivity

– What will SMAX be?– What will UMAX be?– What about slot time?– Maximum cable length?

Maximum Segment Length

• Assume RG-6 coax– How many taps?

Number of Taps

• Number of taps does not include NAP connections.

• No node 0 in ControlNet.• To keep it simple, let’s use taps as nodes 1 to

26.– NAP nodes 27, 28, 29– Total taps = 26

Segment Calculation

• 1,000 meters – [16.3 meters( Number of taps – 2)]

• 1,000 meters – [16.3 meters( 26 – 2)]• 1,000 meters – [16.3 meters( 24 )]• 1,000 meters – 391.2 meters• Maximum segment length 608.8 meters

SMAX

• Scheduled traffic nodes 1- 12• Nodes 1-10 currently used• Nodes 11 and 12 future scheduled

– Unused nodes = slot time• SMAX set at 12

Determine UMAX

• Nodes 13 - 26 unscheduled traffic• Nodes 27, 28, 29 for NAP

– RSLinx drivers for personal computer node addresses MUST be set at 27, 28, or 29.

– RSLinx default for 1784-PCC personal computer interface default = node 99.

• UMAX must be set at a minimum of 29.

Efficient Network Configuration

1 99

UMAX =29SMAX= 12

Node 10 Node 11 & 12

future scheduledNode 27, 28, 29 for NAP

Future Network Expansion

• Recalculate segment length?• Reconfigure SMAX?• Reconfigure UMAX?• Reschedule network using RSNetWorx if any

scheduled node is added or modified.

Reschedule Network

• It must be done whenever a scheduled node is modified or added to the network.

• Network configuration is scheduled in RSNetWorx for ControlNet.– Part of saving new network configuration– ALL processors on network in program mode– Referred to as optimizing and rewriting

network configuration

Rescheduling and the Keeper

• The ControlNet communication module at the lowest node number is called the keeper.– Should be node 1– For ControlLogix 1756-CNB(R)– Keeper like a traffic cop

• Directs traffic on network and synchronizes nodes– Newer CNBs have multi-keeper capability

Multi-keeper

• Older CNBs are single-keeper networks.– Newer CNBs support multi-keeper.– If there was a newer CNB at node 1 and also

at node 2, node 1 would be the keeper and node 2 would be a “back-up” keeper.

– If node 1 fails, node 2 would take over network traffic control.

– In single-keeper systems, if the keeper fails, all network communications are lost.