Register 15 contains a return code indicating
the success or failure of the request. Possible return codes (in decimal)
are:
- 0
- The environment is no longer prepared for a sequence of calls.
- 4
- The function code is not valid.
- 8
- The indicated environment was already active; no action taken.
- 16
- The token is not valid.
- 20
- The token was not used in a start_seq call.