SN65HVD73: UART to RS485 using SN65HVD73

Hi Eugen,

1. So first I would suggest looking at the SN65HVD70 instead of the 73 - the difference is speed. Since your speed is slow and the SN65HVD70 can support the low baud rate we generally suggest using only a part as fast as you need. That being said - you can use the SN65HVD73 - it would still work but its just best practice to reduce high frequency energy content where possible. This is because the 73 has a faster rise/fall time on the differential bus leading to more high frequency energy in the signal - even at lower baud rates. 

2. Since you are using UART - the "R" pin (pin 1) needs to also be pulled up to 3.3V (a 4.7k to 10k is usually what we see for this type of application). If you don't do this the controller could send interrupts as UART defaults to a "high" state - so the default state needs to be high. When you switch to TX mode (since the DE and /RE pins are shorted) the R pin is high impedance - this is really "why" the "R" pin is pulled up even though it is a push-pull output - it specific to the protocol used. 

3. The default setting of the transceiver is in TX mode - which will default to the highest power state of the transceiver. It's generally better to start in "RX" mode as that will minimize power usage of the device - basically only be in TX mode when absolutely necessary. Since this is a full-duplex device - there really isn't much operational risk to do this - but ideally this isn't the best usage of power. 

Beyond that everything else more or less okay. You will cut down the max bus distance with the 10 ohm series resistors and you could possibly see some signal reflections - but since the bus is short and data-speed is low the risk is relatively minor - i.e. I wouldn't be too worried about it working but it might not be the most optimal operational conditions. That being said  it should be noted that the characteristic impedance of the cable and or trace between nodes should also be approximately 120 ohms. 

Finally - both ends of bus should be terminated with 120 ohm resistors. Realistically at the baud rate you are working with the only thing that is going to reflect most likely is the high frequency energy content during signal transitions - but with both ends terminated and a relatively shorter cable - they should be very small. 

Please let me know if you have any other questions and I will see what I can do!

Best,

Parker Dodson