Simple 485 unidirectional and using digital outputs

Projects Networking, Protocols, and Devices
1

Hi there,
I was testing a circuit that I think is Ok, but not totally sure.
The idea is simple turn on/off a rele wich is about 50meters away from the board. I heard about the RS485 protocol and build the circuit below.

I test it at 5meters and works, but probably should get a longer twisted pair.

The questions are:

  1. Do you notice something wrong in the logic/circuit?
  2. Anyone use/do something similar or have another idea?

Think it's for some simple domotics like turning on a lamp using only one board and a few dollars.

Oh... and replace the led for something differnt

485.jpg
485.jpg1280×484 100 KB

2

That looks fine apart from the terminating resistors, they look a bit low to me. You only need them if your signal changes faster than the time it takes to reflect a signal down the line. This can be calculated from the length of the line and the speed of light and the velocity factor of the wire normally about 0.6 of the speed of light. The time it takes the signal to travel down the line and back has to be shorter than you will change the signal.