ControlNet

Introduction

ControlNet is an open industrial network protocol for industrial automation applications, also known as a fieldbus. ControlNet was earlier supported by ControlNet International, but in 2008 support and management of ControlNet was transferred to ODVA, which now manages all protocols in the Common Industrial Protocol family.

Features which set ControlNet apart from other fieldbuses include the built-in support for fully redundant cables and the fact that communication on ControlNet can be strictly scheduled and highly deterministic. Due to the unique physical layer, common network sniffers such as Wireshark cannot be used to sniff ControlNet packets. Rockwell Automation provides ControlNet Traffic Analyzer software to sniff and analyze ControlNet packets.

Architecture

Physical Layer

RG-6 coaxial cable with BNC connectors, though optical fiber is sometimes used for long distances. The network topology is a bus structure with short taps. ControlNet also supports a star topology if used with the appropriate hardware. ControlNet can operate with a single RG-6 coaxial cable bus, or a dual RG-6 coaxial cable bus for cable redundancy. In all cases, the RG-6 should be of quad-shield variety. Maximum cable length without repeaters is 1000m and maximum number of nodes on the bus is 99. However, there is a tradeoff between number of devices on the bus and total cable length. Repeaters can be used to further extend the cable length. The network can support up to 5 repeaters (10 when used for redundant netwoks). The repeaters do not utilize network node numbers and are available in copper or fiber optic choices.

The physical layer signaling uses Manchester code at 5 Mbit/s.

Link Layer

ControlNet is a scheduled communication network designed for cyclic data exchange. The protocol operates in cycles, known as NUIs, where NUI stands for Network Update Interval. Each NUI has two phases, the first phase is dedicated to scheduled traffic, where all nodes with scheduled data are guaranteed a transmission opportunity. The second phase is dedicated to unscheduled traffic. There is no guarantee that every node will get an opportunity to transmit in every unscheduled phase. Both the scheduled and unscheduled phase use an implicit token ring media access method. The amount of time each NUI consists of is known as the NUT, where NUT stands for Network Update Time. It is configurable from 2 to 100 ms. The default NUT on an unscheduled network is 5 ms.

The maximum size of a scheduled or unscheduled ControlNet data frame is 510 Bytes.

Application Layer

The ControlNet application layer protocol is based on the Common Industrial Protocol (CIP) layer which is also used in DeviceNet and EtherNet/IP.

An End User's Experiences

ControlNet's scheduled message system requires that the media be robustly designed and meticulously maintained. A break in the media invariably causes a SNAFU. The network is checked with a handheld device known as a "Network Checker" in conjunction with a DSO running at 100 MHz or higher. The media itself (coax) can be checked with a handheld device known as a "Media Checker". It must NOT be placed on an active network. Its purpose is to check for opens, shorts, impedance problems, etc. It can be used to identify the distance (in feet) from the Media Checker to a short in the cable. In Rockwell PLC systems, the network is configured via RSNetworx For ControlNet. By opening this program the network is scanned and a graphical screen is populated with each node on the network. Each processor has a "scanlist configuration" which shows its connections to remote racks, frequency drives, and any other devices that communicate with it. This program performs for remote racks essentially the same functions as the I/O configuration in the local rack. In the I/O configuration table of Logix, analog cards can be assigned block transfers. In RSNetworx, ControlNet analog cards in remote racks are assigned scheduled messages in blocks of words. Processors communicate with each other via "produced" and "consumed" tags. A processor's produced tag, for instance, may be 50 words. This block will be broadcast for any other processor to consume. Likewise, the same processor may consume 50 words from another processor. Data in this manner can only be produced by one source, but can be consumed by many. Such use of produced & consumed tags require that the network be "scheduled" at some point. ControlNet supports conventional message (MSG) instructions to send data between PLCs as well but as it happens during the unscheduled portion of the network update, those messages don't require scheduling. In those systems utilizing the Logix family of processors, specific mapping of produced and consumed tags is not required to be performed in the RSNetworx for ControlNet software. In both PLC and Logix family systems, the RSNetworx software "schedules" the network after first determining that the pending connections to the various devices can be facillitated with projected bandwidth calculations offering the programmer the option of changing some of the characteristics of the devices before the network scheduling takes place. When a network is "scheduled", thwe configuration is flashed into the memory of those devices on the network that ar "keeper configurable". That is to say, they have the capability to retain the network's configuration with all of its connections (I/O, produced/consumed tags, etc.) A keeper on a network is NOT a master. It permits devices to rejoin a network when they are removed provided they were on the network during scheduling. ControlNet 1.5 allows for multiple keepers on the ControlNet network where the device with the lowest node number is the designated "active keeper". Should the active keeper be removed from the network, the role of active keeper is passed to the next highest node number of a device that is "keeper capable". This is what allows the original active keeper to rejoin the network when it is returned to the network. It is recommended that keeper capable devices occupy low node numbers starting with node 1. (Node number 0 is not available for devices placed on the network.)

External links

• ODVA website
• ControlNet International website
• ControlNet Networks and Communications from Allen-Bradley