Trouble communicating over RS485 between MAX485 and SP485

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1

I have created an RS485 network between two microcontroller as shown below.

[FT232] ->[MAX485]<~Twisted pair~~~~~~~~~>[MAX485] ->[MSP430G2553]

(Transmitter) ---------------------------------------------------------->(Receiver)

operating at 9600bps with 120 Ohm termination resistors at both ends .communication occurs perfectly in this case.

If i change one of the chips to SP485(Similar to MAX485) from exxar as shown below the circuit stops working(MSP430G2553 UART do not receive any data).

[FT232] ->[SP485]<~Twisted pair~~~~~~~~~>[MAX485] ->[MSP430G2553]

but if i use SP485 on both sides as shown below the issue is resolved and communication occurs perfectly.

[FT232] ->[SP485]<~Twisted pair~~~~~~~~~>[SP485] ->[MSP430G2553]

I have checked the data sheets for MAX485 and SP485 they are similar.

FT232 is operating at 3V signal levels.

MSP430G2553 is operating at 3V signal levels.

In all the cases termination resistor value is 120 ohms.

I am just sending a single character through the link.

Have anybody experienced a problem like this?

2

What is Vcc of the MAX485 and SP485?

3

Check the polarity of the data leads. Some vendors use +/-, some use A/B.

The standard calls for the B lead to be + and A to be -.

4

What is Vcc of the MAX485 and SP485 -5V

polarity of the data leads is correct

5

What voltage level conversion are you using to translate from the 5V levels output by the RS485 receivers? If you have nothing then it is possible to induce latchup in the 3V devices and destroy them. Even if you don’t get latchup the excess current demand could alter device operation.

You show only a twisted pair between the two systems. That should be shielded twisted pair with the shield terminated to both system grounds to control common mode voltage.

6

What voltage level conversion are you using to translate from the 5V levels output by the RS485 receivers?

I am directly connecting the DO and DI outputs of both MAX485 and SP485 to MSP430.

I am not experiencing any problems when i am using MAX485 on both sides or SP485 on both sides(no latchup system works fine tx and rx works ok).

Problem comes when i am using like this

[FT232] ->[SP485]<~Twisted pair~~~~~~~~~>[MAX485] ->[MSP430G2553]

7

You show only a twisted pair between the two systems. That should be shielded twisted pair with the shield terminated to both system grounds to control common mode voltage.

I am using a 30 cm long twisted pair wire for testing in the lab.The grounds for both the Transmitter and reciever are taken from a common point.