GeorgeLiu 
Hello, I am manage to make the UART2(pin3 and pin4 on CON4) on NanoPi working but failed. First of all, from the kernel boot log, i can find log as: [ 1.112134] s3c2440-uart.0: ttySAC0 at MMIO 0x50000000 (irq = 74, base_baud = 0) is a S3C2440 [ 1.662891] console [ttySAC0] enabled [ 1.668520] s3c2440-uart.1: ttySAC1 at MMIO 0x50004000 (irq = 77, base_baud = 0) is a S3C2440 [ 1.677397] s3c2440-uart.2: ttySAC2 at MMIO 0x50008000 (irq = 80, base_baud = 0) is a S3C2440 [ 1.686251] s3c2440-uart.3: ttySAC3 at MMIO 0x5000c000 (irq = 98, base_baud = 0) is a S3C2440 so i think the uart are initialed. I did double check the RXD2 pin got uart data from host by scope, but my test utility can not get data from ttySAC2(this utilty works with UART3, so we can exclude the software issue). Secondly, I try echo command to send data to UART2(by command "echo xxxxxxx > /dev/ttySAC2")， no data can be observed on scope(tested on ttyASC0 is OK). According your schematic, the TXD2 is GPH4 and RXD2 is GPH5. i also try export these two GPIO on linux sysfs, but i still can not control this two GPIO(gpio228, gpio229) by gpio sysfs. Please help!