Hello,

I am testing my rs485 communication thru a 60 meters of cable, it seems working at 9600 baud rate. I have one master and one slave.I am using SN75176. I have put 1k resistor between A and B pins. There is a 4,7k from A pin to +5V and 4,7k from B to ground. My maximum communication distance will be about 60 m. Do you think this is the optimal circuit, or what corrections can be done?

I also want to ask, how I can apply noise to this 60 meters cable for test.

Thank you very much.

 

Thank you, I will correct the resistors. I will write the results here.
Also I will write the results after I apply the noise to the circuit.

 

If it is single direction, the 120 Ohm should be at the receive end, if it's bi-directional you need termination at both ends. You could terminate both ends anyway to minimise noise & transients.

The other ultra-critical thing that many people miss is that a common ground is needed between all drivers and receivers. The safest thing is to use screened cable, or other cores in the same cable - CAT5 ethernet is useful for that, use one pair for data and another for ground. Failing that, you can use other grounds as long as the voltage difference never exceeds +/- 5V at any point along the bus cable.

For improved noise immunity, you can modify the termination to two 56 or 68 ohm resistors in series, with a small cap (1nF?) from the center point to chassis ground.
That helps cut down induced noise spikes that could exceed the common mode range of the drivers/receivers, without affecting the fundamental balance or impedance of the system.

 

So I should use one twisted pair for a and b, and one for ground for a better solution. Is there a cable type, that only includes two twisted pair cables and one ground? Because as I see, CAT5 cable include 4 twisted cables some of which I don't need.
Thank you.

edit: I am noting down the advanced noise immunity tip, I will try it by making some manual corrections on the pcb, I may make correction in the future progress.

edit2: You have written 1nF?, the capacitor should be 1nF or that is changable by another condition. (This may be a dumb question, but I should be counted at circuit design ).