Hi
Please check the following application note, almost all the peripherals for the RS08 are the same than the S08, so you can use this document as good start point
https://www.nxp.com/docs/en/user-guide/HCS08QRUG.pdf
software (this is old, but use it as reference)
https://www.nxp.com/docs/en/user-guide/QE128QRUGSW.zip
also check
HS908SH8 - SCI module wrong data transmited/received
https://community.nxp.com/thread/302646
The RS, SH and the QE has the same SCI module
I hope this will help you
Take care