Problem with 485 Driver and Interrupt

I am interfacing a rs485 Transceiver to an ARM microcontroller (LPC2104). I have the write and read enable pins tied together as one is active high and the other low. The problem I am having is, when I bring the pins high enabling write, my microcontroller RX pin is driven low which triggers an interrupt. Conversely, when I bring the pins high to read, the RX line is driven high which the UART interprets as a stop bit and I get a false 00H input. I have tried to disable the UART1 interrupt both in the UART itself and in the VIC. The problem is that as soon as the interrupts are re-enabled it is triggered. When the 485 driver is set the read the bus the RX pin of the 2104 is brought low this, I believe, is detected as a start condition and is the first false interrupt. When the driver is set to read the RX line is brought high. I believe this is detected as a stop condition and presents a false 00H as having been received.

Any thoughts would be appreciated.


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R Soennichsen
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Perhaps you should not have the lines tied together, to allow for boundary conditions.

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