Wow, this is a bad algorithm! If someone else finds himself trying to understand Epson's terrible low-level communication manual, this is how the check-sum is done:
- The checksum base is 30 30 30 30
- Sum in hexadecimals all of the data package (for example, 02+89+00+00+1C+80+80+1C+00+01+1C+09+0C+1C+03 = 214)
- Then separate the result digit by digit, if its a letter add 1 to the value (for example B2 would be 2|1|4).
- sum it to the checksum base number by number starting from right to left (this would be a checksum of 30 32 31 34).
Note: It works perfectly, but for some reason the examples I posted above don't seem to match so much. They are all the printers response, but slightly after it got a hardware problem and had to be reformatted by technical support, so maybe it got fixed.
I hope it helps somebody somewhere.