View single post by Ingo
 Posted: Wednesday Oct 23rd, 2013 04:53 pm
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Ingo



Joined: Sunday Jan 21st, 2007
Location: South Africa
Posts: 557
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Perhaps I can add something to the formula.

If we consider that there is no congestion on any of the interfaces then the RS232 interface will be the quickest.

As Ident explained, the RS485 bus message is translated to RS232 at 9600bps, if I remember correctly, at this point it is ready for sending on the wire. Because things are hardcoded you have a few variables. Some of these are internal chip buffering and then most importantly you have the delay associated with clocking each of the bits out on the line at 9600. Add all these up and you get the absolute minimum delay that can be achieved from an output.

If you look at the Ethernet module you have the added delay of converting the RS232 signal again to Ethernet and then add it's own transmit delays.

In all honesty, all these combined should be fairly insignificant. The biggest contributor to perceived 'slow response' is the sensor itself - if it's a PIR device then it has to properly 'see' movement before it sends a command on the bus. Then you have your response latency, by this I mean your PIR trigger a response, the response does something and set's <something>.

I give you an example on Cbus. I have a Comfort PIR, first it takes some time to 'see' movement, once movement is detected there is something called 'debounce' delay to make sure there isn't multiple triggers for one event. Next, Comfort detects this and fires a Response. More processing is done to execute the response. Inside my response is a Cbus command that is fired over to the UCM/Cbus. Now we start the whole process again, the Cbus network is also waiting for the complete command to be clocked out on it's network, it then has to activate either a relay or dimmer depending on the command sent. All this takes time and doesn't happen instantaneously.

If you explain what you want to achieve perhaps we can help but to answer your question on which is faster? RS232 or Ethernet? Then the answer is RS232.

The quickest test you can do is to connect a door switch to an input and a relay to an output. Note the debouce time of the input zone type and then write a response to switch the Output On|Off as you press the button. You will find the response close to immediate once you subtract the debounce delay, which you can set to a different value at your own risk of false triggers.

Ingo

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