ocr: C64 CIRCUIT THEORY RPa 5.36 CO55RD 6526 A DCC 4 RZ" EFLE a 30t 3.3K K laooghen IM KPB A28 BOORA ROKUBLTREIN U2 (DDUC DDFF) AUr The Serial Interface and User Port Circuits. The Serial Interface. U2 is a Complex Interface Adapter (CIA). Parallel port signals PA3-PA7 control the serial bus inter- face. PA3 is the Attention (ATNI output. This signal is inverted by U8 before being transmitted to a device on the bus. PA4 is the Clock output. Data transmitted from the C64 to a device on the bus IS synchronized by this clock signal. U8 inverts the output PA4. PA5 is the data output. U8 inverts this ou ...
