Overview

Multi-Drop connectivity products have become integral parts of modern process control, SCADA and traffic control networks. They provide an effective-and inexpensive-means to link multiple devices shared by a single communications media, typically fiber optic cable.

TC Communications specializes in Multi-Drop connectivity. It offers a unique blend of special Redundant Ring Ethernet Switches, Fiber Optic Modems, Multiplexers and Broadcast & Receive Devices that is unmatched by any single vendor. Together or singly, these Multi-Drop products provide users with maximum flexibility, ease of use and installation, and reliability.

TC's Multi-Drop fiber optic modem & multiplexer products offer two features that are immensely beneficial: a sophisticated self-healing ring topology and an intercom channel. The self-healing ring immediately detects cable faults or even downstream device failures. It automatically re-routes the communication path to insure uninterrupted service. The intercom channel offers real time voice communications between two or more nodes at the same time.

In addition to Switches, Modems and Multiplexers, TC Communications offers a Broadcast & Receive device that is specifically designed for SCADA applications (fiber or copper cable) using a star topology. It broadcasts and receives messages from the host controller to “addressed” Remote Terminal Units.

TC Communications Multi-drop fiber optic modems and multiplexers offer several numerous benefits that enhance network flexibility for installation, troubleshooting and expansion. These benefits include:

• The ability to add additional channels to the network at any time without affecting the system.

• The ability to re-configure the network at any time, e.g. moving hardware devices to different rooms.

• The ability to adapt to different power sources or interfaces.

• Separating broadcast & receive signals to avoid interference if the self-healing function is activated.

• Anti-Streaming. Anti-Streaming prevents a single node failure (e.g. "babbling") from bringing down the entire network.

Polled-Response Communications                                      

TC Communications' fiber optic modems and multiplexers work with multi-drop "polled- response" networks that typically use RS-232, RS-422 or RS-485 interfaces. Polled-response communications are often used in SCADA, traffic control and process control systems.

In these networks, the Master Modem (Host CPU/Controller) “polls” each Slave Modem (Remote Terminal Unit [RTU]) in sequence via a pre-designated address. The "addressed" Slave then responds back to the Master. If the Slave (RTU) does not respond after a designated time or a certain number of attempts, the Master (Host) will break off communications and go on to the next Slave.

This response time depends on the length of the message, the sophistication of the message and the number of sensors attached to a given RTU. Although there isn’t any theoretical limitation on the number of nodes or RTUs a multi- drop system can support, there are practical limitations. Practical limitations include the type of connecting media, type of transmission devices, and tolerance of the software.

TC Communications Multi-Drop Fiber Optic Modems and Multiplexers are available in four basic configurations:

One or two Master units and at least two Slave units are required to form a self- healing ring. For a dual ring configuration with one Master unit, Ring A is the primary ring and Ring B is the backup ring. The fiber optic signal travels clockwise in Ring A, and counterclockwise in Ring "B."

During normal operation, when there is not a fault condition, the data broadcasted by Master #1 travels through Ring A to Slaves 1-4. After receiving the broadcasted data, the Slaves put it back on Ring A and it goes clockwise back to the Master unit. During a fault condition, such as a cable break between Slave #1 and Slave #2, Slave #1 will detect the downstream fault and wrap around Ring A to Ring B. In the meantime, Slave #2 will detect the breakage upstream, and wraparound Ring B to Ring A.

More on Self-Healing Ring Configurations....

In most multi-drop networks it is imperative to maintain normal system operation when a unit fails or a cable breaks. A Self-Healing Ring (also called Dual Ring) topology maximizes network reliability by enhancing fault tolerance and network flexibility.

A Self-Healing ring consists of two data paths, Ring A and Ring B, with each data path running in an opposite direction to the other. Under normal operation only the primary data path (Ring A) is used. When there is fault (cable or failed device), the data path will turn around before it reaches the fault or failed unit and use the secondary data path to complete the link. In this manner, the data path to the remainder of the units will remain intact.

An effective self-healing ring should offer immediate detection of cable faults or device failures. Any failure should result in automatic re-routing of the communications path to insure uninterrupted service. Power redundancy is also highly desirable.

It is important to note that all Self-Healing Ring topologies are not alike. Vendor offerings can vary greatly in terms of function, features and sophistication. (For example, some vendors do not separate broadcast and receive channels. As a result, activating the self-healing function can cause these channels to interfere with each other).

Self Healing Ring with "Dual Master" TC Communications offers an extremely reliable "dual master" self-healing ring configuration. This innovative configuration virtually doubles the reliability of a single Master because it can recover faults or failures in both rings in two different places.

Thus, integrity can still be maintained despite double faults in Ring A and double faults in Ring B. During normal operation, Master #2 is set in a "THRU" mode via a DIP switch at the front panel. When a double fault (all four fiber optic cables in a fault condition) occurs (see figure at right), Master #2 is switched back to "MASTER" mode.

In "MASTER" mode, the broadcast messages are sent through Master #1 and Master #2 at the same time. The responses from the slave units are also received by Master's #1 and #2. In extreme situations where both rings are broken in two separate places, both Master units poll the Slaves at the same time, but through different "half" rings.

Flexibility

Communications Engineers will appreciate the flexibility of TC's Multi-Drop fiber optic products. This flexibility can be evidenced in at least three ways:

1. The ability to add additional channels to the network at any time-without affecting the system.
2. The ability to re-configure the network at any time, e.g. moving hardware devices to different rooms.
3. The ability to adapt to different power sources.

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Page Last Updated: 12/15/2006