Recent versions of NMEA0183 use RS422, which is the same as (or at least very similar to) RS485 but with only one transmitter. +/- 5V nominal, differential. Older versions used basically RS232, up to +/-15V on data line, referenced to ground.

RS 422/RS485 levels should be at least +/-200mV to register as 1 or 0. Nominal drive 5V.

Idles high, same as RS232 and UART.

In practice I have seen (open circuit unless otherwise stated):

• Vesper XB8000 AIS puts out +/- 3V. Loaded by “MAX490” board - no change. By iMac usb board: +1.8V, -1.4V.
• Cheap iMac colour USB → 485 dongle: +500mV, -3.5V.
• Cheap UART to RS422 board, with chip marked MAX490: +/- 4.3V open circuit, dropping to -4.1V, +1.2V when connected to XB8000 receiver.