I am designing a product (prototype) that consists of many RS485 networked nodes. They will share a common bus.
My network will be small on size and doesn't need a very high baud rate. RS485 recommends termination resistors, which I want to add. Now, I wouldn't like to request the user to add these resistors at the ends of the bus, because the user is not expected to be tech-savyy. I was planning to add a termination resistor inside each module in the bus, thus making redundant resistors. In my prototype, I want it to seem that you can "just plug them in"
Standard setup:
simulate this circuit – Schematic created using CircuitLab
intended setup:
This means each module has two jacks: one to connect to the bus, and other jack to chain the following module into the bus aswell.
So maybe a higher value resistor could result in a sort of progressive terminator resistor? More modules–>lower overall resistance between A and B.
I have also heard that you can use this standard with only one terminal, if you don't expect noise. Is that true?
Best Answer
The termination is required because of the characteristic impedance of the wire used for the bus ...typically a twisted pair. Putting multiple termination resistors on a short cable will load down your signal.
Read this application note.
If you have in and out connectors on your boards, then you should supply a Terminator that the user plugs into each end of the physical bus. That way you do not need terminators on each board.
Alternatively you could put a resistor on each board and a switch, which would be set (turn the resistor on) on the leftmost and rightmost board on the bus. This is easy to describe to a user.