how fast is the C-Bus ?

The cbus transmission speed depends on the SIZE of the commands.

First - we will assume lighting commands, which are small.

There are 2 cases to consider.

Case 1. How many SINGLE lighting commands can you send?

Answer: A lighting command is either 2 or 3 bytes long, depending on the command. The message header, routing, checksum, and so on is BIGGER than the command. But in this case, sending about 20-30 commands / second is possible. It takes a lot of very very very careful software coding to achieve this. Using a PCI as the interface, you will most likely in normal cases achieve 10-15 commands/sec by trying very hard and being very careful. It is possible to do more, using a PCI, but the techniques to do this are unusual and require a lot of effort. Clipsal don't do that extra effort.

Case 2. What about scenes?

In the case of scenes, several lighting commands can be packed into a single message. For example, a single message might be able to control between 4 and 10 lighting groups at a time. The number depends on the network routing, and the commands. Using 6 at a time as an average is a reasonable guess.

In this case, the overhead in sending the message is the same as the case of the single control, as above. But you can control 6 times as much per message.

Because the messages are a bit bigger, they take longer. Typically these big messages, in a scene, might take twice as long to send. So by being really careful, you can send about 8 to 10 BIG messages per second. But each of those BIG messages controls 6 groups on average. This means you can switch or ramp approximately 60 groups / second.

Doing this is difficult, and typically you won't get this performance. But being able to switch or control about 15 to 25 groups / second using big messages (as used in scenes) is reasonable by writing code to drive a PCI, and doing it very carefully. Being very very very careful and using some obscure techniques you can increase this to perhaps 20-30 groups / second, or maybe even a bit more. However, this is theoretical and clipsal dont do this.

SUMMARY:

It depends on what you want to do. If you cover raw messages / second, then it depends on the message and its size. 10-15 messages / second is achievable with a lot of care, provided those messages are short.

If you look at what the messages do... that is HOW MUCH CAN I CONTROL, then the messages get bigger but each message does more. 15-30 control actions / second is achievable.

These numbers are all theoretical, and to achieve them in practice needs careful writing of software.