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((((((( P L C U P D A T E N E W S ))))))) PLCS.net -- Your Personal PLC Tutor Site FREE PLC programming info... in plain English! http://www.plcs.net==================================================Hi DharamvirIt's been a while since we last talked about our newest protocol friend... FOUNDATION Fieldbus. Well this issue is going to pick up where we left off. We'll quickly see some of the different communication modes it uses and what each does.Is it right for your application? Depends on what your application is... of course. Read the below and the previous issue and you should have a good, basic understanding of the bus. Then you can make an informed decision. So, read on!But wait, before we start let me remind you about our PLC training dvd series. All the information you need to learn plc programming is available on 5 dvd's. The cost can't be beat either. Far less expensive than a class, and you'll have it for review when you need it. Now's a good time to start learning...You WILL learn how to program PLCs. (It's also good for trainingone or many people without sending them to a class...)See the details and special pricing here:http://www.plcs.net/videok.shtmlEnjoy the topic!Phil MelorePLCS.net, LLCphil@plcs.nethttp://www.plcs.netNow on to the topic... FOUNDATION Fieldbus - Part 2!================================================== * * * S P E C I A L O F F E R * * *--FREE Shipping--If you're looking for a good book to help you learn plc programming,please consider ours!Help support the site and get FREE *Priority Mail* Shipping too!!!!You won't regret it.Click below to see the details.http://www.plcs.net/book1k.shtmlRead the book 1000's of programmers trust... but not at the typicallyhigh textbook cost :-)==================================================++++++++ ===================================================****** FOUNDATION Fieldbus... A real 'standard' bus - part 2=================================================== Let's quickly get back to where we left off before and find what makes FOUNDATION FIELDBUS 'work'.Remember that we can use this bus like a tree. We run a trunk line and can tap into it as needed by adding branches or 'spurs'. The maximum branch length can be 120m but it depends on how many branches we have as well as the total number of devices per branch. More devices is going to mean less length.The maximum length of the trunk is also going to be limited to about 1900m... assuming we use 18AWG cable. Thinner cable is going to give us a less maximum length. 22AWG only gives us 1200m, for example. Please note that 1200m is still quite far... wouldn't you agree? Oh yeah, don't forget to add terminators to each end of the bus.So, how do the devices on the bus talk with the plc? Good question. Let's review that now...Only one device is allowed to access the bus to talk at any given time. So, we need a way to ensure that is what happens. Enter the LAS (Link Active Scheduler). The LAS controls and schedules any and all communication on the bus. It is the traffic cop, if you will. It controls the bus by broadcasting different commands to all devices. It also continuously polls unassigned addresses so it's possible for us to connect a new device while the bus is running and it will be recognized.On the bus we can have scheduled as well as unscheduled transmissions. Some things that need to be done repeatedly are handled by the scheduled transmissions. The LAS periodically broadcasts a synchronization signal called a TD(time distribution). This ensures all devices on the bus have the same time synchs. Since time to transmit can be guaranteed we can say that FOUNDATION FIELDBUS is a deterministic bus.The bus may have only one LAS. Any device that is capable of becoming an LAS is called a 'link master'. There may be many link masters on the bus but only one can be the LAS. If the LAS fails then one of the other link masters may take over as LAS. Kind of a semi-redundant design.So, on the bus each activity can be scheduled for a particular time. The schedule is based on an offset from a preset time. Based on that schedule a transmission list is generated which tells the system when a specific device can send it's data. After getting the send data message the device broadcasts it's data to all devices that are configured to receive it.The broadcasting device is called the 'publisher' and the receiving device is called the 'subscriber'. So, this is a publisher/subscriber sequence of communication. The LAS issues a CD (compel data) command to the publisher and when it receives the command it publishes the data to all subscribers. The subscribers then read the message. Simple.Since each device receives it's own schedule from the LAS, we can know exactly when any task is going to happen and when specific data is going to be sent or received. Very organized. But what about unscheduled transmissions? Good question. Sometimes data is needed on an 'as needed basis'. This type of data is not time critical. FOUNDATION FIELDBUS can handle that data as well. It takes care of that data whenever there is 'open air' on the line. If the bus is not doing a scheduled task it is free to use.The LAS issues a PT (pass token) command to the device wanting to send. This pass token is then rotated to all devices on the 'live list' of devices needing to send data. Each device then gets the token and has permission to talk. When done, the token gets passed along to the next device on the list. The LAS asks each device not on the live list if they need to be added to the list with a PN (probe node) command. The device sends back a PR (probe response) command if they want to be added. Devices that don't reply are not added, or get taken off the live list.Each 'control loop' accesses the bus only once for a short period of time. So, the bus can therefore be used for many control loops or other things. This adds to our efficiency and shows one of the benefits to a distributed control plan. Good stuff...Well that's enough for this time. Lots of information to soak into the brain. Take a break and then try to reread the above. It will make even more sense than it does now!See you next issue! Learn, enjoy and PLEASE tell a friend or 2 or ...about the site and updates. They will appreciate you for it!Enjoy,PhilPS- Why not visit our PLC Training store if you have a minute: http://www.plcs.net/store.shtml================================================== * * * A N N O U N C E M E N T S * * *If you like what you're reading - Pass it Along!Feel free to forward this newsletter to friends or colleagues (or yourboss) who are interested in the best PLC information and the largestPLC community on the Web today.==================================================(((Ask a PLC question...Get an answer)))Check out our LIVE Interactive -PLC question and answer- section at the site today. Have a PLC related question, ask the community. Knowthe answer to someone else's question there, then post an answer. It'sa very active section with tons of questions and answers. Be sure tocheck out the questions/answers already there. 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