If I want to build a MAC that is able to handle the Token-Bus Link Layer and some portion of the physical driver I would need more information on the specific differences of the required MAC functionality compared to a standard 802.3 ethernet MAC.
I looks to me that the MAP 802.4 usage within the control industry is very rare. And it seems to be to old to be used.
However some still use this approach with a Token in order to get a deterministic behavior compared to standard 802.3.
If someone could point me to some useful information in order to write my own 802.4 interface (VHDL) using an FPGA, this would be very helpful.
Why? What application cares about the internal details of the Link Layer?
The 802.4 standard is public knowledge, as is the 802.3 standard. They are completely different, such that it would not be possible to modify an existing 802.3 controller to operate as an 802.4 device. It is also unreasonable to use an 802.3 controller as the foundation for designing an 802.4 device.
It is rare to the point of non-existence. It's lack of usefulness has nothing to do with its age, however (if that was the case, then *I* would no longer be useful, since I am older than 802.4!); it is due to its lack of advantages vs. the cost of implementation.
There is no determinism in *any* LAN MAC protocol, to the extent that determinism means you can guarantee any specific behavior. This is true for Ethernet, Token Bus, and Token Ring. With any of these protocols, however, you can achieve a probability of acceptable behavior to any level you desire.
As someone who was a member of the IEEE 802.4 standards committee (which, by the way, is now completely disbanded), and was actually responsible for the development of an 802.4 controller implemented in semi-custom and custom silicon, the most useful information I can give you is to go back to the bar, order another round of drinks, and figure out how to use Ethernet in your application.
-- Rich Seifert Networks and Communications Consulting 21885 Bear Creek Way (408) 395-5700 Los Gatos, CA 95033 (408) 228-0803 FAX
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