Discovery response not recognized

[sblair]

Welcome to the forums!

A couple random thoughts of things I've ran into before to also check:

- Could be a timing issue in the response. Make sure everything is within the right timing window.
- You might need to leave some additional time between sending the packet and shutting off the Tx drive. It is super easy to disable TX before the packet has fully been sent chopping off some bits at the end and causing the behavior you are seeing.

Scott

[lensfocus]

Thanks so much for your responses, Scott and ericthegeek..


I found the problem, it was a bad ground on my RS485 driver chip!


I did try adding a delay after sending the packet before enabling the receive, but that was allright, I am waiting till the Tx buffer is empty first.



Anyway, I still have the question about the checksum initialization.


ericthegeek, I understand what you said that 6 * 0xFF is the same as adding the header bytes, but the header isn't supposed to be included in the discovery response checksum! I am now getting a "MUTE" instruction addressed to my UID after discovery after initially setting the checksum to zero.


Thanks again, this is a great forum!

[ericthegeek]

Originally Posted by lensfocus

I found the problem, it was a bad ground on my RS485 driver chip!

Oh good, glad you found the problem.

Originally Posted by lensfocus

ericthegeek, I understand what you said that 6 * 0xFF is the same as adding the header bytes, but the header isn't supposed to be included in the discovery response checksum! I am now getting a "MUTE" instruction addressed to my UID after discovery after initially setting the checksum to zero.

The 6 * 0xFF isn't from the header (the header is 0xFE, not 0xFF).


Each byte of the UID is Boolean OR'd with 0xAA and 0x55.


We've established that (0xAA.or.VALUE)+(0x55.or.VALUE) == 0xFF+VALUE.


If you expand that out 6 times, once for each byte of the EUID you get:


(0xFF+UID_5)+(0xFF+UID_4)+(0xFF+UID_3)+(0xFF+UID_2)+(0xFF+UID_1)+(0xFF +UID_0)


That can be simplified to (6*0xFF)+UID_5+UID_4+UID_3+UID_2+UID_1+UID_0, which is the calculation that's done on that code snippet. Whoever figured out that simplification is clearly smarter than I am.

Originally Posted by lensfocus

Thanks again, this is a great forum!

That's why we're here. The goal is to keep RDM developers on track and to help improve interoperability for everyone.


It's often worth browsing through older threads too, it can help you avoid many of the common pitfalls.

[sblair]

Originally Posted by ericthegeek

Oh good, glad you found the problem.

Whoever figured out that simplification is clearly smarter than I am.

And that is exactly why I don't try to be clever in my code. I'll just end up confusing myself later. I don't recommend trying to be too smart in your code. Eric is one of the more brilliant people I know. If the code is trying to be too clever for him, then it's too clever for most anyone.