Kenneth Lerman wrote: > I think the easiest way to do this is for a point to point network or > even a network with a hub is to use raw ethernet packets. An ethernet > packet has a payload of up to 1500 bytes. That should be large enough > for most things we would want. > > On top of the payload we have around 20 bytes of overhead, including a > CRC. I think we could come up with a simple polling protocol. The EMC > machine would be the master. > > To me, the thing that needs the most thought is how to build a generic > HAL component to handle this. It is the same problem that SWP and I > discussed at Fest regarding Modbus. Ideally, you could have a > configuration file for the driver that specified the devices (in the > case of ethernet if would be by MAC address) and how to map HAL pins to > channels on the device. > > We would need to have a comprehensive set of signal types. There is a > decent amount of work involved in spec'ing this out. Offhand, for > inputs, I see things line single bit, multi-bit word input, signed word > input, counter input, counter that resets on read input. I would make > words all the same length; at least thirty-two bits, but possibly sixty > four bits. The fact that the transmission time is very small makes that > reasonable. > > Enough, for now. I don't think I have the time to really work on this now. > Yeah, that's where I am, too. But, given the right foundation by some open-source group, it shouldn't be all that hard. I'm not sure whether rtnet is that foundation, or too restrictive and specialized for our purposes.
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