Lets say i have some MAX 488 connected to two ATmegas to talk to each other.
If i then want to upload a sketch to them with a FTDI USB-TTL do i need to disconnect
the RS485 chips first?
Maybe one ATmega is on one end of the house and the other on the other end and both are running.
Then i want to reprogram one of them...
You just have to be sure the serial communication of the FTDI can overrule the MAX. An option would be to connect the RO via a resistor to the RX and connect the header for programming direct to the RX. This way, when you plug in the FTDI it will overrule the signal from the MAX. The TX can be connected straight to the max and the programming header.
But keep in mind that the data that the ATmega sends to the FTDI while programming is also send to the MAX an thus to the other ATmega. So the other ATmega must ignore them aka, the protocol you use between the ATmega's must have some aspect that makes it unique so you know what data you did sent and what is garbage (data from the programming process).
Thanks septillion. Then i think its easier to use some jumper