Hi,
The ATMega has TTL/ CMOS level outputs.
You did't say, but I assume the PLC has RS485.
(RS232 is not ued at all, therefore it is unclear why you talk about RS232)
RS485 is half duplex, where you only have one signal (pair) for receive and transmit combined
RS232 is full duplex with one signal for receive and one signal for transmit.
With RS485 you therefore need a signal to select between receive and transmit..
Additionally you need to select the correct baud rate, UART setup (8N1), and you need to implement the PLC protocol in your AVR software.
*****
Your post lacks a lot of informations, therefore it is impossible for us to find out where your problem is.
Klaus