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BUSNET :: Level 1
   


Electrically BUSnet level 1 (physical) can use any media for transmission, currently only RS-485 is defined but RS-232/422, TTL, optical, wireless etc can all be used on one of more links in a network.

The BUSnet physical interface is a simple RS-485 transceiver. Any common transceiver can be used.

Nodes talk to the two adjacent nodes (neighbours) using full-duplex RS-485 links, this is a bi-directional ring topology, meaning that data flows in both directions.

 

The publish/subscribe model should also make the linking of wired networks to other wired networks using wireless links. So a main network in a house can communicate say with others in various out buildings or places where running a wire is not practical.

 

 

   
    Point-to-point, advantages and disadvantages    
   


Given that just about every network known to man is multi-drop the decision to go point-to-point (a ring topology) was not taken lightly, indeed this design was multi-drop for a long time. However I think that overall point-to-point will make for a better and more reliable system.

One may argue that the robustness of a ring system such as this is subject in turn to the robustness of the code running on the processor that directly interfaces with the PHY level as this processor must store and forward data to the next node.

That's true, but it's also true of a multi-drop system and indeed any system that uses microprocessors to interface with the bus. With a multi-drop system a failure in the interface processor can bring down the entire bus, simply by forcing the data levels to a fixed state or by transmitting continuous garbage, with point-to-point these faults can only affect the nodes on either side and even then it should not stop them from functioning as they can still communicate through their neighbours on the other link.

The following sections detail my thinking on the pros and cons of this topology over a more standard multi-drop bus.

Pros

Bus clashes - There are no bus clash issues, this makes the software simpler and in theory more robust.

Mixed data rates – Slow and fast nodes can coexist. Links that are potentially more error prone can run at a slower rate while the rest of the network runs at high speed.

Adaptive links – Nodes can dynamically adjust the link data rate according to the number of errors encountered on the link.

Fault isolation – A single cable or node fault cannot bring down the entire network, it may have no affect at all or may only affect one or two nodes. With multi-drop a short circuit between the data conductors stops all data flow.

Mixed PHY layers – Links can be implemented with any physical transmission type. Currently only RS-485 is defined but there's no reason RS-232/422, LIN, CAN, TTL etc can't be used for the entire network or just some individual links.

Bus length – As each link between nodes is an RS-485 connection the normal parameters apply to it. Therefore distances between nodes can be up to 4000 feet or 1.2k. As there can be as many as 255 nodes there is a theoretical maximum bus length of over 190 miles or 300k. Something I assume will never be tested.

Cons

Expense – Each node requires two RS-485 transceivers and the bus cable needs two more conductors.

Propagation delay – All frames are delayed because each node consumes the entire frame before re transmitting it.

Branches/stubs – Branches and stubs require two cables, you cannot simply T off an existing cable.

 

   
    Cabling    
   

 

BUSnet can use any style of cable, however 8-conductor Cat5/6 cable and RJ45 modular jacks are specified as the default.

Each node has two RS-485 ports known as A and B. Port A must be connected to port B on a node's neighbours and vice versa.


   
 

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