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RDM Timing Discussion Discussion and questions relating to the timing requirements of RDM.

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Old March 17th, 2015   #1
mkoelman
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Default Timing contraints driver switch from controller to responder

Hi there,

New on this forum, and to rdm for that matter. I hope someone can help me to clear up some timing details that I can not seem to find in the E1.20 specs.

I am currently working on implementation of a rdm responder in a LPC11xx MCU.

The issue I am facing is that in the situation of a controller doing a request to
a responder, with an expected response, I am not entirely clear as to what the
timing constraints are w.r.t. the bus (drive) switch from controller to responder.

What I understand from the specs is that after the last stop bit is sent by the controller, the controller keeps driving the bus with MBB for a maximum of 88usec. After this time (or before), the controller stops driving the bus.
I do not think the responder can detect the bus not being driven anylonger, as the biasing circuitry will pull the bus high if undriven (true?).
Does this mean that the responder will always have to wait the full 88usec
before it can initiate a break? If the responder starts its break earlier, we can not garantee the minimum break time (176usec).

Can anyone elaborate? Any help would be most welcome! Thanks.

Meindert Koelman
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Old March 17th, 2015   #2
ericthegeek
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Quote:
Originally Posted by mkoelman View Post
I do not think the responder can detect the bus not being driven anylonger, as the biasing circuitry will pull the bus high if undriven (true?).
In a properly functioning RDM system you never need to detect whether the bus is being driven or not. Bus access is entirely determined by timing.

The biasing circuitry will pull the bus high when it is un-driven, but this can take a while due to capacitance on the wire. That's why RDM devices are required to drive the bus high before they disable their line drivers. This way the bias circuit only has to maintain the bus state, not change it.

Quote:
Originally Posted by mkoelman View Post
Does this mean that the responder will always have to wait the full 88usec
before it can initiate a break?
Response timing is defined by E1.20-2010 Table 3-4 "Responder Packet Spacing Times". A responder must wait a full 176us before it can initiate a break.
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Old March 18th, 2015   #3
mkoelman
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Dear Eric,

Thank you for your swift reply. It is clear now.

Kind regards,
Meindert
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