Here are the digits pairs:

0 <-> 0
1 <-> 8
2 <-> 4
3 <-> C
5 <-> A
6 <-> 6
7 <-> E
9 <-> 9
B <-> D
F <-> F

The whole signal has four bytes, each of which is two hex digits. For example, Stop is:

FE 01 C0 3F

First you must find each digit in the above table (on either side) and change it to the digit that the table pairs it with. So Stop becomes:
F7 08 30 CF

Next you must swap the two digits in each byte, so Stop becomes:
7F 80 03 FC

The first two bytes always start as FE 01 so they always end as 7F 80, so you don't need to redo that part every time. But I included them this time to make the process clearer.