Protocol for reading the Start or stop bits for RS485

[GaryZ21]

What's the protocol for reading the start bit?  Ex: if the comm is set for 8bit, 1 Parity, 2 Stop.   Will the Rx or the driver read every twelve bit and validate the start bit based on the first bit of every twelve bits, from the received data?   Is the sample taken from the 50% duty cycle or is it edge-triggered - a high is on rising edge and a low is on a falling edge?

[Chris_R.]

Hi GaryZ21,

You will not be able to read the start/stop bit the same way as you read the data. Those bits along with the parity bit are handled at the hardware transceiver level and never make it to the driver. The driver only knows if the data is good or bad with the way you set it to do so (parity, etc).

RS-485 transcievers use edge triggering to detect the start bit and oversampling to align the framing of the bits.  Around 10 samples are taken for each bit, so it is typically quite a bit more that a single sample at the midpoint of the bit.

I hope this helps, but let me know if you have any questions!