Well, I can't see anything wrong with the ASM part. It compiles, and looks like it should work. But there are a couple possibilities ....

If you are expecting to receive data from DMX channel 7, it will be in DMXrxbuffer[6], since the first "Device ID" byte (always 0) is discarded.

And, 250k baud is pretty fast for a resonator. You might want to try a crystal with caps.
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