Is there anybody who knows what is this old chip? And where to try to find it? If it help what I know is that it used to do somenting like shift data register or Serial Shift Registers. It has a TTL level signal on its pins, it has 14 pins. You can see it named U1 in the centre of this schematic: <https://www.dropbox.com/s/zqhgar4g7ib5j4z/Xerox820_FDC_Schematic.pdf> https://www.dropbox.com/s/zqhgar4g7ib5j4z/Xerox820_FDC_Schematic.pdf Some pins are labelled on a schematics with: 1=D1, 2=D2, 3=D3, 4=D4, 10=A1, 11=A2, 12=A3, 13=A4, 14=A4, 15=ChipEnable (left to GND), 7=GND, 14=+5Vcc Thanks Enrico