You've been told to Mux the 232. This works great in practice if you have the ability to do it and is easy to implement. The other choice people have mentioned was 422/485 which is a little more difficult to implement but requires the ability to do it as well. The next choice would be to add a brain that acts as a FIFO switch and assigns a priority to one of the messages. I'm not sure what happens to device B when it polls every 500ms and does not get a response, but if you needed a response in a timely manner, I think this method would work. I take it as device C is trying to ask for something or say something within that 500ms time window. You are in control of device C which means that you could potentially re-design portions. Adding an MCU with SPI or I2C or some other comm and 2 UARTS might be a way to go assuming that device C has the capabilities. We'll call the new brain device D (that's right fix problems by adding more! HA.). Device D takes input from Device A, Device B, and Device C. The firmware could easily multiplex the information. The main concept is that you're really reducing the number of 232 lines by replacing one with a different comm medium. Adding Device D also means that your project can be more expandable in future work if necessary. Otherwise, I think Device C could just act as the switch in between? I am just not sure of how are you handling all the data and where the final data ends up. Just an extra thought.