41C setting the default printer to serial via 82164A

[Sylvain Cote]

At power on, the HP-41 is searching for a device class type 2 (printer, aid=32...47) and if found will set the flag 55 and 21.
The 82164A is part of class 4 (interface, aid=64...79) which is not seen as a printer and clears the flag 55 and 21.
This is why you need to switch to MANIO, then SELECT the interface before sending data to it.