The ATmega128(L) has an additional UART compared to the ATmega103(L). The RS- 232 port on the STK501 board has in addition to the RXD and TXD lines support for RTS and CTS flow control. Figure 1 shows a simplified block schematic on how this is implemented.
The UART in ATmega128(L) does not support hardware RTS or CTS control. If such functionality is needed, it must be implemented in software.
This UART can also be used from devices placed in the STK500 board. Simply connect the appropriate port pins to RXD and TXD on the STK501 board.
If no software RTS/CTS flow control is implemented, a jumper shorting RTS and CTS will ensure correct communication with an external application that uses such flow control.