Simple Resol data bus to Raspberry Pi serial port

[Dave L]

The Resol data bus a simple, single polarity, nominally 8 V, serial data stream. It is not a differential signal like RS485 or even a differential signal with an 8 V bias. vBus is the protocol used in the data stream carried by the Resol data bus.

To feed the 3.3 V serial port on a Raspberry Pi the Resol data bus needs to be level shifted and some protection of accidental reversal of the bus provided. Here is a simple circuit:

R1 and R2 form a potential divider for the nominal 8 V data bus. The specification document gives a maximum of 8.2 V that equates to 2.94 V at the transistor base. The transistor is simply acting as a switch and also prevents a potentially Pi damaging voltage reaching the serial port if the polarity of the bus is reversed. R3 ensures that the Rx D line is pulled low when the transistor is off. The transistor is not critical, almost any small signal, general purpose, NPN, transistor will do, the BC239 was the first I found in the junk box.

If you only want to snoop on the data bus why faff about with USB adapters, far more complex circuits or trying to persuade a MAX485 board to work and that still requires a level shift?

Disclaimer: Works for me. I have tried the half dozen or so possible input mis-wirings without the Rx D line exceeding the Pi's serial input limits. YMMV...

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Cheers
Dave.

PS. Sorry if this appears twice I thought I'd posted it earlier this morning but I can't see it in the topic list.

[Dave L]