Ocelot wiring


Staff member
Someone in the chatroom asked me this question earlier, and the more I think about it, the less I am sure I even know the answer. Do you have to daisy chain expansion modules, or can you use a 'star' topolgy, with every module going back to the ocelot.
Ideally, they should be daisy chained. A star topology for RS-485 is not very good as far as signalling and signal reflection goes, unless the modules are physically close togehter.
This is not an Ocelot-specific answer but I believe it holds. I think the Ocelot uses RS-485 to communicate between modules. By definition, RS-485 should be used in a daisy-chain configuration (or a chain with short stub-links going to each module).

In practice, you can often use a star configuration. But it pretty much comes down to trying it to see what happens.

One approach you can take is to simulate a star configuration while really using a daisy chain. The Ocelot uses one twisted pair for bidirectional communcations (two wires). Say, for example, you are using cat5 and the blue pair of wires for your communications. You can run from the Ocelot to the first module using the blue wires. At that module, connect the green wires in that cat5 to the same terminals in the module - this brings the signal back to the Ocelot. Now connect those green wires to the blue wires of another cat5 and run this to the next module. You have effectively created a star configuration as far as wiring goes but it looks like a daisy chain to the Ocelot and its modules.

You can continue this to add as many branches to the star as you want.

The RS-485 standard allows for very long connections (up to 4000 feet at low baud rates). I've easily run this configuration (but not with an Ocelot) to 1000 feet out and back along the same cat5 at 9600 baud with no problems (I didn't test a faster baud rate). Of course, that 1000 feet was still in the box but it shouldn't make that big a difference (world's longest patch cable).