I just finished writing a driver for the monoprice 4x2 switcher with RS232 control.

The cable required is odd. pins 2,3 are swapped but all other pins especially 4 DTR are connected straight through.

The DTR pin is important. Without DTR on the monoprice provides no status back. RTS,CTS,DSR,CD have no effect and can be connected or disconnected, they should be wired straight through if connected.

Finally the status is continuously sent back while dtr is asserted.

If the switcher is not powered on, it still sends status back, however it won't switch. Since the status includes whether it is on/off you can look at the status and if it is off send the power toggle command 0x10, 0xEF, 0xD5, 0x7B to turn it on.