libmodbus | Open-source library for MODBUS TCP and UDP | TCP library

When linking with an external library, you need to pass in the name of the library with the -l option:

You have a header that defines a static bool LOGGER_DEBUG_MODE =true;. If you include that header in multiple C++ files then each file will gets its own copy of that bool.

In your main.cpp you aren't using R_DEBUG so the copy of that bool (which presumably comes from including logger_adc.h ) is indeed unused in that file.

Possible solutions are:

You should make it so you only have a single copy of that bool (declare it in the header with extern and define it in a single C++ file.

Use build defines instead of runtime checks

etc

You can use the socket.close(fd) which takes the socket descriptor and closed it.

Did you install the libmodbus-dev? You should try this: sudo apt-get install -y libmodbus-dev.

See v3.1.6 and v3.0.8: libmodbus docs

In the traceback, a get_string function is raising a UnicodeDecodeError because it can't decode some text from ASCII.

In the current source for elftools, this line has been replaced by

There is a very easy solution to your problem: just don't set MODBUS_RTU_RS485, quite likely you don't need it.

This mode is actually a workaround for devices without automatic (hardware) direction control. As you know, Modbus RTU works over a half-duplex RS485 link (only one device is allowed to talk while all others must be listening only), and hence requires an additional (to RX and TX) signal to control what device is writing to the bus at all times (direction control).

So you would only need to set MODBUS_RTU_RS485 if your device lacks this feature, which is nowadays quite unlikely or if you are building your own transceiver. Most devices based on the FTDI chip, in particular, should have this feature since the chip itself has a TXDEN (transmit enable) pin. See here for more details and a trick to expose the TXDEN signal to a non-default pin.

It is when you don't have this feature (one frequent scenario is when you want to use the embedded UART on your Rpi for Modbus over RS485, implementing your own transceiver) that you need a software (or hardware) workaround. And that's where MODBUS_RTU_RS485 should come handy, repurposing the RTS flow control signal. Unfortunately, most serial drivers (including ftdi_sio, the one you are probably using) don't support this mode (refer again to the link above).

Luckily, there are workarounds to the workaround: see here for a complete discussion. You can also take a look at this answer where I explained how to set up libmodbus with support for toggling the direction on the bus using a GPIO pin on a Rpi (also applicable to most SBCs, I've used this method successfully with a Pocket Chip computer, for instance).

You can find more background on this issue elsewhere: here and here.

It seems your slave does not have any holding registers at address 0, and hence it's complaining with the illegal data address error.

You need to look at the Modbus register map you have and use the correct Modbus function from libmodbus accordingly. For instance, if you want to read your variable RLY_pulseOnWidth you have to use:

This is what works for me with Python 2.x: