SN65HVD3086E: Data communication

Hi Sandra,

So RS-485 distance is limited by data-speed. 

The SN65HVD3086 is spec'd at a max data-rate of 20Mbps (with RS-422 max at 10Mbps) - are you using max RS-422 speed?

If that is the case - you will not get a driver to go that fast + 300 meters:

Based on general RS-485/RS-422 performance  at around 100 meters you won't get much more than 10 Mbps. With when you are seeing the issues with communication - it seems to be about correct with the above graph (if running this device as full data-rate) 

So I don't think it is an actual driver issue - it is that most likely you are operating at a speed that can't support that distance. If possible can you try decreasing the data-rate and see if you see an increase in distance - I'd imagine adjusting the data rate to between 1Mbps and 5Mbps will get you higher distances.

One more thing to note is to understand the DC loss of the cable - as if the cable is resistive it could also degrade max distance - but I do think the largest issue is most likely how fast you are moving data through this line.

Another issue that could be limiting max distance is how the device is set-up on the schematic. 

This is a full-duplex device so it has a Rx bus and a Tx bus. The Tx bus (pins y and z) don't need the termination resistor as it is only recommended at the most distant receiver from the driver -however I am not sure if there is another driver communicating to the A/B bus of this device in  the system - if there is and this is the last Receiver on the line - it should be terminated. The end point of the Y/Z bus (I don't think its on schematic) should also be terminated as this will help mitigate reflections which could also degrade max distance.

So in conclusion:

1. If you are using max RS-422 data rate (10Mbps) or higher you will not get 300 meters out of the system. I'd suggest if possible dropping down the data-rate (1Mbps - 5Mbps would be a better place to try) ; if that's not possible - then the RS_485/RS-422 standards will not work for this application as this isn't a TI transceiver issue - this is a RS-485 / RS-422 standard definition. 

2. Ensure cable résistance is low to see how much DC loss is going to be caused by that cable - as this could also effect max distance. 

3. I am not sure if anything on this schematic needs to change (since not all connections are shown) - however, the last receiver in the RS-422 chain should be terminated to help mitigate reflections - so the Y/Z bus at the last receiver should be terminated and the if the A/B bus is the last receiver on that bus line than it should also be terminated. (greater than 100 Ohms but should be as close to characteristic impedance of transmission line to mitigate reflections) 

Please let me know if you have any other questions!

Best,

Parker Dodson