Differences

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--- public:cbuspublic:start [2020/01/22 15:24] – [Introduction] grovenor
+++ public:cbuspublic:start [2020/01/22 15:24] – [Hardware requirements of the BUS] grovenor
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 CBUS is a Layout Control System based on the CANBUS. A description of the system can be found on our public webpages.
 cbus.php and cbus2.php
-==== Hardware requirements of the BUS ====
-**The choice of CAN.**\\
-The CAN bus (Controller Area Network) was developed by the Robert Bosch company in the 1980s for use in motor vehicles but has since been applied to many other types of machinery including aircraft and medical scanners to name just two. It became an open international standard as ISO 11519 in 1994 and a higher speed version as ISO 11898 in 2003. It is now used in virtually all modern motor vehicles so there is a wide applications base and ‘off the shelf’ components are readily available. When we looked at CAN, it seemed pretty much the ideal for a LCB. It was intended for relatively infrequent transmission of small amounts of data between devices for control purposes where response times and safety were paramount. Unlike the more familiar ‘Ethernet’, it was not intended for shipping large amounts of data between computers. Another advantage of CAN was that it was already in use for a LCB by Zimo, so there was proof it worked in a model railway environment.
-The data rate chosen for CBUS is 125Kbps. This is one of the defined CAN rates which go up to 1 Mbps but there is a trade off against cable length. 125Kbps allows lengths of up to 500 metres, good enough for even most garden layouts. The wire should be a twisted pair but doesn’t need to be screened. Only the ‘standard’ CAN frame is used.
 ==== The messaging scheme. ====
 After much debate, we settled on the ‘producer-consumer’ model at least for layout control. For those used to the idea of sending specific messages from A to B – a ‘source-destination’ scheme, this is a very different concept although widely used for industrial control systems. Imagine changing a switch on a control panel. This creates an ‘event’. A frame is sent on to the bus which contains no source address, no destination address, no information, just an ‘event’ number. The node sending an event is called a ‘producer’. All nodes capable of processing an event are ‘consumers’. Every consumer on the layout receives the event. What the consumer does with the event will depend on information already in the consumer. In effect, a consumer has to be taught what to do with any event it has to act on and to ignore events that are not relevant.