Hi all,

This is my first post so I hope that I get a good responce, sorry its so long.

Anyway, I am looking for some practical advice about noise reduction / protection of a distributed 12vdc power supply.

I have designed a RS485 data collection system. the basic setup is a master unit, mainly all digital using a PIC18F4520 MPU with a few other bits and pieces such as a RTC, MAX232, MAX483 etc.
The master is supplied with a plug in 'phone type' 12vdc  supply, which will also charge a Li-Ion battery.

I have a number of 485 nodes (Max 32) all plugged into the master. Each node is based around a PIC18F1220 with a MAX483 for the 485 comms. Each node will collect 4 voltages on its ADC and then send the results back to the master when requested. The node will need 3.3v and (Maybe) 5 volt locally to run the PIC etc.

I am using CAT5 cable for the 485 bus going from the master to each node in turn.

I intend to (Not done yet) use one of the spare twisted pairs to supply power to all of the nodes from the master. Proberly 12V, but this can be any voltage really.

Now, I need some advice with the decoupling and bypassing of each node. The total cable length will be less than 300M and I don't want problems with power supply noise or interferance bewteen nodes.

I have read the 'POWER SUPPLY NOISE REDUCTION' paper by KEN KUNDERT and found it very interesting.

However, from a practical point of view what power supply circuit should I have for each node?

I was going to use a good 3.3v linear reg and the usual caps, but I think I should have some sort of low pass filter or decoupling circuit using an inductor and cap before the input to the regs?

Also, should I have some other type of protection such as maybe using transzorbs etc to crowbar any larger spikes etc.

And one last thing, for the PCB layout  of each node I was going to use one side of the board as a ground plane and the top for +ve (As mush as possibe).

Any suggestions or advice about this would be much apprieciated. Where to put componets and what values etc! I have a basic understanding about decoupling and bypassing but never really needed to go into any further.

Thank you for taking time to read this long posting.

Kind Regards

Paul