Async serial module
This module allows communication between host chip and a PC. It is necessary in umbilical version of the system where it is used to bootload the host chip. It can also be used to inspect or modify content of external SRAM, onboard SPI flash memory or MMC, as well as to exercise all host nodes accessible to ether messages sent from a PC.
Node 708 runs code that continuously listens to the asynchronous serial line, if a start bit can be detected, and to DOWN port, whether node 608 attempts to write to it. When data are received from the async line they are converted into 18-bit words and written to DOWN port. Similarly, when data are written to DOWN port by node 608, 18-bit words are cut into bytes, and sent via the async line LSB first.
Node 608 is an ordinary ether node with the exception that it is the first node to be loaded with ether during bootloading via async line, and from this node other host nodes are loaded with ether code.