Problem with RS485 interface....

[sanddune008]

rs485 10k

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 contains noise 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(yellow w/form) and master output(blue w/form)......

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

 

[Arkham00]

Re: Suggestions

(1) do not use R1 and R6: just connect the GND of the MAX485s directly to 0V
(2) change R2 and R3 with much higher values from 10K to 100K
(3) why microcontroller HB5 uses TX only while HB7 controls RE/TE of the MAX485 transceiver?

Regards

 

[sanddune008]

Re: Suggestions

(1) do not use R1 and R6: just connect the GND of the MAX485s directly to 0V
(2) change R2 and R3 with much higher values from 10K to 100K
(3) why microcontroller HB5 uses TX only while HB7 controls RE/TE of the MAX485 transceiver?

Regards

Thanks for your time....

(1) do not use R1 and R6: just connect the GND of the MAX485s directly to 0V

what would be the advantage of removing Resistors R1 and R6?...

I added these resistors to prevent excess current from flowing to GND which may introduce noise..............Or is it more than that?.....

(2) change R2 and R3 with much higher values from 10K to 100K

These resistors were add to put a check on the floating conditions that may arises if the wire is broke..........isn't ? correct me if i am wrong.

 

[Arkham00]

From your schematic, it seems that R1 and R6 are in series with the GND pin of the MAX485. If this is true, it's wrong because in this way the resistor interferes with the power supply of the IC. Usually in RS422/RS485 connections, you don't put resistors on GND but simply connect the GND of the two remote circuits. Only in case the two remote GND are connected to different EARTH points, you may need a resistor to limit current flow but in this way, the difference of potential made by resistor, may break down the RS422/RS485 chips. It's much better to keep remote GNDs far from earth.

About R2 and R3, I know what they are for, of course. Because RS485 is bidirectional, it's a good idea to put the two resistors near each node but this may consume more power (imagine a network of 32 nodes, each with a 10K to GND!). Many new IC don't need that resistors. From the MAX485 datasheet: "the receiver input has a fail-safe feature that guarantees a logic-high output if the input is open circuit". So Max485 does not need the pullup/pulldown resistors.

 

[sanddune008]

Thanks for your time........

Removing the resistors R1 and R6 and adding a capacitor b/w Vcc and GND helped me get a smoothen the slave i/p waveform.....