I don't seem to be able to get this library to link. I get four similar error messages
for example:
Error: L6218E: Undefined symbol eMBRegCoilsCB(unsigned char*, unsigned short, unsigned short, eMBRegisterMode) (referred from BUILD/LPC1768/ARMC6/mbed-modbus/mbfunccoils.o).
Would very much appreciate any help in resolving this.

Hello, thanks for the project. But now MBED 6 I could not run this project. Can you please open a new branch for MBED 6?

Is this repo still maintained?
I found this problem when I tried to use this modbus lib with serial and parity even:
https://forums.mbed.com/t/unbufferedserial-sending-with-parity-even-does-not-work/22377/2

The portserial.cpp does not regard the parity argument. I had fixed this, but still no success. Then I found that the enable_output function starts the initialization again and did not restore the parity/wordlength settings, so that the modbus Ack was sent with wrong framing.

I think the the intention of using the enable function is to enable/disable the according ISRs, but the function also does all init stuff.