How to calculate the checksum algorithm with data captures

Question

I have an obsolete control system that I am trying to write an interface into our new control system. I have spent weeks capturing data and figuring out the addressing, polling commands, point addressing and payload. I have everything figured out except on how to calculate the checksum on the packets. Below is a small sample of the data - I believe the first 2 bytes are the checksum, bytes 3/4 - address, bytes 5/6 - command, remaining bytes are the payload:

0e00 0801 1280 //to controller

0e20 //from controller

0e10 0801 1200 8010d00d9b0a19120a27375f01010412071f0e0a0512aa10495349474854000000000000000000000000000000000000000000001400ff0e0e00000000 //from controller

0e40 //to controller

0f00 0801 1285 //to controller

0f20 //from controller

0f10 0801 1200 0000000000000000000000000000000001010a0512aa105f1400ff //from controller

0f40 //to controller

1000 0801 1285 //to controller

1020 //from controller

1010 0801 1200 0000000000000000000000000000000001010a0512aa105f1400ff //from controller

The checksum appears to be out of order when viewing the data captures, but that is the order that it was captured in.

I don't know if it matters or not, but I am writing the interface in Java.

I appreciate any help that can be provided ... many thanks in advance.

Answer 1

The first byte starts as 0E is then 0F and then 10. This looks something like a sequential message id, not part of a checksum.

The second byte has a pattern to it too and, taking into account the message lengths, I'd interpret it as follows -

00 => request to controller
20 => acknowledgement from controller of the request 
10 => response from controller
40 => acknowledgement to controller of the response