When a SLAVE is sending a packet and the last byte has been loaded into the UART then you will typically get a UART interrupt (or a TI indication status) at the end of the last character. It is common for those inexpereinced with RS485 transmission to turn off the transmitter driver at the time this last interrupt happens. Unfortunately the TI status and corresponding Interrupt normally (for almost all derivatives) will happen in the center point of the stop bit time. If the slave takes action to disable its driver while the stop bit is still in sequence this level will be removed and the stop bit time truncated. This could in some instances cause the RS485 bus to begin another bus transaction with a new start bit while the other truncated stop bit time was still active. Of course this problem would be casued by improperly designed RS485 protocol drivers in software. It is also easily corrected in software.

Michael Karas