To use serial port E, there are samples. I only know the rs485 sample in the dc 9.62 package, but I am sure that sample carries over to 10.x. I use the programmer's port on the 3k and serial E without a problem. You have to toggle transmit and receive for it to work right.
Like I said, check the samples and you will see how to do it. You can also set more than the speed (i.e. data format, etc).