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| RS485 unterminated |
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| rakeshm55:
Hi, For a Board to Board communication UART communication , I plan to use a RS485 signal level Drivers. The trace length will not be more than 50cm. RS485 is expected to operate inpoint to point mode. UART speed is expected to be of the order of 12Mbps and Industrial grade temperature range is expected. In this case inorder to reduce power consumption can I use RS485 in un-terminated mode. If the termination is avoid reflections then my bit period is of the order of 50ns, so a 5ns round trip delay can be accommodated. Will there be significant reflections for 50cm trace length?? If terminated with a 120Ohms resistance the total power consumption is of the order of 200mW(3.3V RS485). Any recommendations to reduce power for RS485. |
| rstofer:
At the bit frequency you want, I doubt that the design will work reliably without terminating the lines. Maybe, maybe not, but more likely not. So, design the resistors in and leave them out for a trial. This article does suggest that you would only need to terminate one end in the case of One Transmitter - One Receiver. Check the signal integrity with a scope and see what you think. https://www.maximintegrated.com/en/app-notes/index.mvp/id/763 |
| Giaime:
In my experience, we were running 20-30cm of 3Mbps RS-485 in an highly noisy envinronment (6kW AC/AC IGBT converter) and it didn't absolutely work w/o proper termination :-\ Maybe for DC power consumption you can try AC termination, it's documented in various RS-485 appnotes. |
| mikeselectricstuff:
I'd be surprised if it didn't work unterminated with only 50cm You may want to bias the lines to avoid glitches at start/end of packet. if it is terminated, you could reduce power by running the transceiver at reduced voltage, or higher value termination resistor |
| mikeselectricstuff:
Might be worth looking at LVDS as an alternative. |
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