Problem with RS485 interface....

Hi All,

Prj Description:
I am designing a hub which receives i/p's(max of 4 inputs) from actuators and slaves connected to the hub(output). When an actuator input is received Master(Hub) will process it and sends a common command to all the salves connected to its output....based on the command each slave will do the necessary.


MAX485 is connected to the UART.

Now the problem is the slave receives data which is noisy and hence displays junk day to the LCD........

I am sending "Hello world" at baud rate : 384000


Can anyone please verify my schematics.....which is attached.

Also attached is my slave input to micro controller (yellow w/form === at Pin R0 ) and micro controller output to MAX485 (blue w/form)......

Thanks in advance...........

Why did you include 100 ohm resistors in the ground leads of the MAX485? This will confuse the MAX device as its input from the microcontroller is referenced to GND which is not common to both. I have never seen a current limit resistor in the GND pin of a interface chip like this.

How long is the interface cable? Try using a really short cable, aka 1 to 3 feet and see if the waveforms improve. RS485 should really have twisted pair wire. Make sure that the wires are paired correctly. Using 1/2 of two different twisted pairs can cause real big problems.

Ground on the cable is required. The ground is needed to properly reference the two differential signals. While many people do not understand this, RS485 is at minimum a three wire connection.


The 680 ohm pullup and pulldown resistors seem too small to me. I would try using 4.7K or 10K. You can look at :

**broken link removed**


which gives details on RS485 wiring.