Mmmmh...Crunchy!
WARNING! I'm still experimenting...Don't get used to the flavour just yet.
;; serial port to write to
(setq *sout* (make-serial "pseudo0"))
;; the other end
(setq *sin* (make-serial "pseudo1"))
;; write some
(write-byte-seq *sout* (flexi-streams:string-to-octets "Hello World!"))
;; read some
(flexi-streams:octets-to-string (cerial:read-byte-seq *sin* 12))
"Hello World!"
Or more 'idiomatically':
(with-serial (sout "pseudo1"
:baudrate 115200
:parity :PARITY-ODD)
(write-serial-byte sout 42))
For a full description of the API see the wiki pages of the future!