2009/5/5 Xiaofan Chen : > On Mon, May 4, 2009 at 10:27 PM, Olin Lathrop = wrote: >> Xiaofan Chen wrote: >>> You may be right here as well. But I am not convinced CAN is simpler >>> than RS485. >> >> Both can use the same physical layer. =A0RS-485 ends there. =A0CAN hardw= are >> provides peer to peer bus arbitration, collision detection, retry, and d= ata >> integrity check. =A0The most the hardware provides for RS-485 is a UART = and >> then you're on your own. =A0Using the CAN hardware sounds a lot simpler = than >> writing the code and dealing with the many pitfals to roll this yourself= in >> firmware. >> >> Inventing your own bus arbitration protocol is trickier than most people >> realize if they haven't tried it themselves or thought it all the way th= ru. >> You have to assume bit errors can happen anywhere, including in vital >> control information. =A0Nodes must be able to to down without locking up= the >> protocol, etc. =A0It gets even more complicated if you want true peer to= peer >> without a master controller. =A0Normal RS-485 hardware driven from a PIC= UART >> isn't set up to detect collisions. =A0After you think it all the way thr= u, >> you'll realize CAN isn't so complicated after all. >> > > Ok. I see your points here. However, he is also facing problems trying > to use Star topology. Given that and if you think it all the way through, > maybe Ethernet + US$10 Ethernet Switch is the way to go. ;-) > > -- > Xiaofan http://mcuee.blogspot.com > > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > As long as you avoid fast edges by using a slew rate limited driver or a LPF and limiting baud rate then running RS485 unterminated and with star etc configuration works fine. We've been doing it on systems for 10-15 years so far without problems. (19k2 baud rate, run length up to 100m or so and tested to much longer lengths). RP -- = http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist