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Im trying to connect HP39GS scientific calculator to Wemos D1 arduino. It has TTL RS-232 serial port. Pinout is somewhat: Vcc, TX, RX, GND.

Looks like HP39GS protocol for serial port is not pure RS-232, didnt find any documents for it. The HP48 has one article (HP Journal article about interfacing HP48) which suggests that also HP39GS might use some kind of proportional negative voltage levels. Receiver pulls its TX line up (senders RX), and sender pulls its TX line up before sending. Can these be seen as RTS and CTS signals? Note that these can be done in whatever order, since the serial port connection can be used between two calculators.

When HP39GS pulls the line up, looks like voltage is around 3V, which was unexpected. This suggests that there is no voltage level problems. But when connecting Wemos D1 / Arduino board to circuit, the voltage level of TX line drops to half, around 1.6V. When disconnecting ground wires between HP39GS and Wemos D1, it can be noted that there is 3V voltage level potential between the two grounds -- this messes the TX line voltage I suppose. Why is that, and how can I fix the problem?

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