This RS232 interface was published as a DIY project in a special edition of the the German magazine: CPC Schneider International (Sonderheft, #3-1986) and also in a later issue.
As the standard CPC series does not include a RS232, this interface was very handy in order to communicate with other computers(eg. IBM PCs)
Technical specs
Based around: MC6850, and MAX232 Ports used: F8DC - Control register, F8DD - Data register.