Let me add an additional comment about ring communications.

I was contracted some years ago (1983 / 1984 timeframe) to help a very large restaurant chain in the USA evaluate the design of a distributed processing restaurant management system. This system used 8048 microcontrollers (the older brother to the 8051/8052) that had two UARTS attached and implemented in to a DUAL ring communications system. The communications was cabled over low cost point to point plastic fibre optical cable at 1200 baud. I can tell you that the company making this system had a boatload of problems keeping the communications system working. Any time they added a new type of device to the chain or upgraded the EEPROM firmwware in one of the existing modules they went through a whole process of having to "make it work again". It may have been a large part of bad system software design that led to the problems but they had a whole team of people working on it an still the comms were ultra unreliable.

Michael Karas