Code:
download and execute Main LOOP $10
<6D 10 F0 36 80 03 56 00 00 80 20 3D 20 00 FF FF 7E 02 ..
>6C F0 10 76 00 73 00
I think that before the pcm handles control to the download routine it must send 73 as a response that means all is ok.

78 is usually send in multi message answers that require more time. The meaning should be wait I am busy, not ready yet.

To clear the meaning of most of the responses you will have to go through the code again.

Another thing I noticed that the response came from $18, If you send a frame to $10 and get a response from $18 there is something wrong.