MODBUS bus and stub configuration: component troubles

[troy]

Hi!

I'm working as a summer trainee in company X and was given a task to research whether a MODBUS sensor/actuator system could be implemented using a bus and stub configuration (btw. is this same thing as multidrop system?) or not. This seems to be possible since the communication interface to be used is RS-485.

My question is: what kind of component is needed to split the "trunk cable" or the "main cable" (I'm not sure about the right translation) so that the connection between the "main cable" and the sensor can be made? What I'm hoping to find out is, if this is the cheapest way to go about the whole business. Daisy chain configuration is also a possibility or not?

I hope my question was clear enough. I really don't have too much idea what I'm talking about here... Being a newbie and all... If you need more info about the system, I'd be more than happy to provide it.

Rergards,

Miksu

 

[Harald Kapp]

That depends on the type of connection you're using.

Assume you have a bus wire without dedicated connectors. You then can use srew-terminals on the attached devices. Just split the bus wire where the attached device sits, strip the ends of the wires and put the corrresponding ends of the bus wire into the screw terminal.

Assume you have a connector type like D-SUB. You then split the bus wire and solder it to the respective pins of the connector in the same way as shown above.

Assume your device is some distance away from the bus cable. You then may need a small junction box to attach wires for a tap.

Note that the last solution suing a tap is unfavorable. The tap inserts a discontinuity in the bus wire and at high data rates this may cause disturbances by reflections. Try to avoid taps and always keep them as short as possible if you can't avoid them.


Harald