Problem with RS485 interface....

[sanddune008]

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.

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


Can anyone please verify my schematics.....which is attached.
Also attached is my slave input(yellow w/form) and master output(blue w/form)......

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

 

[JimB]

See my edit of your schematic.

Remove the 100Ω resistors in the ground lines of the MAX485s, they are not needed, just a very bad idea.

Add 0.1µF capacitors between Vcc and Gnd at the MAX485s.
All circuits need adequate decoupling of the supply lines.

JimB

 

[3v0]

There is a problem in the RS485 schematic, See
https://www.electro-tech-online.com/custompdfs/2009/02/microsys_art_RS485.pdf figure 1. This setup has worked well for me.

One one end of the network you need 3 resistors as on your schematic. On the other end you need a single resistor between wires A and B. This resistor absorbs reflections. The best value for the two resistors between A and B depends on the network. Use you scope to find a value the works best.

It seems odd to me that you have tied one node in the recieve and one in the transmit state. That does not allow the reciever to ACK or NAK a msg/packet.
3v0

 

[sanddune008]

Thanks for your time.....
Hey JimB............Thanks.