I'm using the ATmega2560 and had terrible trouble getting this to transmit on the K-Bus.
I got it working accidentally when poking around with a scope probe and shorting the Rx pin to the CS pin. This took Rx high during Tx ( I had a pull up on rx but this did not seem to work). I double checked it was definitely the Rx pin going high which made this work by tying it to 3.3V, which also worked (obviously not very useful if you want to receive data).
This previously worked with SoftSerial, but I guess the Rx line got pulled high in the driver, possibly..
I will have to also connect a digital line to the rx pin and set it to output mode and HIGH when I want to transmit, otherwise set it back to input mode.
Any ideas what has gone on here, the datasheet for the MCP2004 says Rx is an input only?