Nios-II SOPC UART delivers wrong bytes

did you try 50 Mhz as clock ?

I use Quartus 5.0 Nios 5.0, no OS

clock 50Mhz and uart baudrate 115200

I changed circular buffersize in avalon_uart.h (from 64 to 2048 bytes)

We use Hyperterminal and ymodem file download to nios with

no retransmits (uart works ok).

I had tow pitfalls with RS485 (older projects) I can remember,

1. use 1k pullup,pulldown (BIAS resistors) to VCC and GND to get a defined

level if no device is active on the bus.

2. to switch of the rs485 tranceiver it is sometimes dangerous to use

a "transmit register empty" interrupt, (perhaps data is still present

in the transmit shift out register, tx still running)