Electronics > Beginners
RS485 unterminated
rakeshm55:
--- Quote from: Daixiwen on July 06, 2018, 07:46:38 am ---I'd recommend an AC termination too.
--- End quote ---
What will be the value of capacitor to be used for AC termination??
Today I did try using RS485 receiver unterminated for 10Mbps UART and it worked, distance was hardly 4 inches - Room temperature 25C.
Daixiwen:
Use a scope to see how much margin you have. You could have more noise in the final application.
If you transceiver datasheet doesn't recommend any value, basically you just need a capacitor that is big enough so that the time constant RC is way over the transition time. But too high a capacitor and your consumption will increase, so experimenting may be necessary to find the correct value.
Kjelt:
RS485 describes a 120 ohm resistor IIANM , what happens if you put in a larger value say 1k or 10k ?
The impedance changes but is it not better to have something than nothing ?
mikeselectricstuff:
At high rates, I'm not sure an AC coupled terminator will save you much - as data rates approach reflection times, the cap is going to be conducting most of the time. Might be tuneable to optimise for a particular baudrate and cable length.
JS:
--- Quote from: Kjelt on July 06, 2018, 11:08:29 am ---RS485 describes a 120 ohm resistor IIANM , what happens if you put in a larger value say 1k or 10k ?
The impedance changes but is it not better to have something than nothing ?
--- End quote ---
With a much higher resistance as you said the reflection would still be there, like 90% for the 1k and 99% for the 10k, so not helping much...
Even if the round trip is much shorter than the bit period, you left the receiver end with 10k, you have a reflection of 99% when it finds the receiver, then, when it finds the transmitter you have a reflection of the same amplitude in the opposite direction and this goes back and starts again. The thing takes a while to extinguish and you might encounter problems. You could try out and find what's going on, probe the line and see what happens at both ends.
If one is transmitting and one receiving you could increase the termination and the source resistance by adding a series resistor to the source output, if both are transmitting and receiving this won't work as now you have a series resistor to the receiver and signal amplitude would be low.
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