Need RS485 driver that have high imunity to noise

Hi Fernando,

We'll look into this and get back to you. For now, I can say that shielded twisted pair makes a big difference with respect to noise immunity. The shield acts as a Faraday cage, and the twisted pair allows external noise to couple to the lines as common-mode noise, which gets rejected by the differential receiver. If you are using two untwisted wires, the noise will couple in as differential signals and the receiver won't filter it out.

Thanks,
Jason 

Hi Jason,

I know about twisted cables, but a similar equipment with CANBUS works without twisted cable. Is CANBUS drivers better imunity to noise?

Best regards

Fernando 

Hi Fernando,

To your point the RS482 (and CAN) will work without twisted wires. Its just more likely that noise will be seen as a bit since the noise will not couple as evenly onto both wires if they are not twisted. It may cause a differential voltage accross the wires and be seen as a bit to the receiver.

I will check with our systems engineer to see if any particular receiver on one of our RS485 devices is better suited for this application and try to follow up with you by the end of the day.

Thanks,

John

Hi Fernando,

I think I will need to learn a little bit more about your system to be able to make a good recommendation.

• Is your untwisted pair of bus wires terminated?
• How long is your bus?
• What speed are you planning on using?

We have different transceivers that are designed for different applications. For example:

• The SN65HVD05 - high differential output voltage (2.3 volts)
• SN65HVD1176 - Low input capacitance (capable of high speeds)

Let me know more about your system and I can try and give you a more catered recommendation.

Thanks,

John

Hi John,

Thank You for the support.

My applications consist of a 1 master and max 40 slaves. It works at baud of 38400 in halfduplex communications

The wires must be not twisted cables.

The environment is noisy than a Frequency voltage converter is used to drive a 3 phase motor

The problem is that sometimes the microcontroller make reset trough watchdog. It seams that it freeze

Have a test equipment run with twisted cables and a most of time works. But i need that it runs

with normal cables. The terminations resistors are 220 Ohm

The RS485 drivers that i use are MAX487 case SOIC08. I will ask for a few samples of SN65HVD05 like your sugestion

to make tests. If you can recommend anyother device please let me know

Thank You again

Best regards

Fernando

Dear John,

I will get a few samples of both devices to make tests and after that i give you feedback

Thank you very much

Best regards

Fernando

Hi Fernando,

As you mention that you work with motor I recommend to look at isolated solution, such solution  will allow you to isolate common mode noise from motor.

For this purpose you could use either isolated RS485 ( like ISO3082) or use standard solution with digital isolator like ISO7241/42.

Hi Fernando,

Were you able to test the samples? Please let me know if we can close out this thread.

Thanks,

John