Commodore Modem Switch and Jumper Settings 1600/1660 Modem Switch O A Originate Answer 1660 Modem 310476-01 and 310476-06 Jumper* Closed Open Enable carrier detect Disable carrier detect (Required for Plus/4) * Solder trace between two square pads on bottom of modem 1670 Modem 310476-02 Switch Down Up 1 Multiple-line connection Single-line connection 2 Normal use Analog loopback test 3 Enable carrier detect Disable carrier detect (Required for Plus/4) 1670 Modem 310476-04 Switch Down Up 1 Originate Answer 2 Enable carrier detect Disable carrier detect (Required for Plus/4) 3 Enable speed detect Disable speed detect 4 DTR always on Enable DTR detect