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Old April 28th, 2010   #4
Task Group Member
Join Date: Aug 2008
Posts: 375

> What is the maximum time of t between the falling
> edge on the command port and transmitting on the
> responder port?

Section 4.2.2 permits each inline device to delay the data by 88uS.

> If it's a non-discovery packet with a break, the device
> must shorten the break by no more than 22 μs, so that's
> an upper limit on this time.

An inline device can receive a byte, hold on to it for 88uS, then send it out. It can also shorten the break by 22us. The two are separate, and an inline device could conceivably do both to some degree.

> 1. All devices should ignore preamble bytes of
> 0xff as well as 0xfe. This would allow the
> first 8 μs low period of the discovery response
> to be shortened by an arbitrary amount.

It is good practice for a controller to ignore corrupt preamble bytes (as you propose). But in order for an inline device to comply with the standard it must shorten the preamble byte by a very small amount of time, or by an entire byte. The requirements in your proposal #2 are all included in the document, just not in a single list.

I worked on an inline device that accidentally corrupted the first byte of a discovery response. It confused many controllers.

Last edited by ericthegeek; April 28th, 2010 at 12:17 PM.
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