A GPS module sent garbage instead of NMEA: it was RS-232, not TTL

I had a GPS module with no markings apart from the maker’s name (Skylab) – most likely an SKM51. According to the datasheet it uses TTL levels, so I hooked it up through a cheap FT232RL USB-UART adapter. Data arrived regularly and repeatably, but instead of readable ASCII NMEA sentences it was garbage.

What I checked

• a second adapter – same result,
• different baud rates and serial settings – either no data or garbage,
• 3.3 V and 5 V logic (adapters with a jumper) – no difference,
• the TX↔RX wiring – checked many times,
• a Raspberry Pi instead of the adapter – the same repeatable garbage,
• several antennas – all behaved the same.

The clue: a negative voltage

I measured the voltages on the original device that used to talk to this module: VCC +5 V, TX 0 V, RX −5 V. A negative voltage on a data line is the signature of RS-232 – generated by the charge pump in a MAX232-style chip. The standard allows levels from ±3 V to ±15 V, so −5 V is within spec. And the datasheet did say the module comes in two variants: RS-232 and TTL.

That explains the symptom: RS-232 is inverted relative to TTL – a logical one is a negative voltage – so a TTL adapter reads inverted bits. The data is repeatable but unreadable.

The fix

I used an RS-232-to-TTL converter, plus, for convenience, a cable with a breakout to screw terminals so I would not have to cut the original lead. I also had to bring +5 V to the GPS module and tie its ground to the USB ground. After that, the data was readable.

Lessons

• Repeatable but unreadable data at the right baud rate is a classic sign of inverted logic or wrong voltage levels.
• Before trusting the datasheet, measure the idle voltage on the data line: TTL idles high (3.3 or 5 V), RS-232 idles negative.
• Another possible cause: some GPS modules output the maker’s binary format and have to be switched to NMEA with the maker’s tool.

Source: my thread on elektroda.pl (in Polish) – Moduł GPS Skylab SKM51 i chińskie przejściówki: nieczytelne znaki ASCII w formacie NMEA (September 2020).