[Noisebridge-discuss] what could we do with one of these babies..

[Jesse Zbikowski]

On Sat, Apr 17, 2010 at 8:47 PM, Joel Jaeggli <joelja at bogus.com> wrote:
> On 04/17/2010 06:09 PM, Jesse Zbikowski wrote:
>> EtherCAT is a hack on Ethernet which allows you to address 100's of
>> devices in microseconds (as opposed to milliseconds).
>
> Well, the normal lower latency bound for ethernet is the size of the
> packet at a given rate and the latency associated with the serdes if
> there is one in the path. at 10Gbe rates, a single 1500byte pdu can be
> clocked out at a rate of every 1.2us, which goes up to 7us at 9k frame size.

Right, the transmission latency of a single Ethernet packet is very
low. This is what makes it a good basis for EtherCAT. Indeed in the
case of a single transmitter and a single receiver, I expect the
advantages of EtherCAT would be moot.

Once there are multiple senders and receivers on the bus, the story
changes. With standard Ethernet if two senders talk at the same time,
you have a packet collision. This forces them to wait for a random
amount of time, retransmit, and repeat -- as many times as necessary.
So the worst case is theoretically unbounded badness; you could be
sitting around all day waiting to talk, or at least milliseconds.

EtherCAT on the other hand uses a ring topology, where every device
talks in turn and there is no bus contention. Hence our jitter (delay
between when we expect a signal and when we actually get it) is low;
less than a microsecond. Deterministic behavior is the key to
realtime.

Certainly there are systems which can use normal Ethernet, especially
if the number of devices is low and realtime requirements are soft.
Determinism is hard to fake though.

http://en.wikipedia.org/wiki/Ethercat#Synchronization