There are 2 basic versions of the 11/53 CPU. They both have 2 serial ports. 1 has 1/2 meg on board the other has 1.5 meg on board. They are both available with Sbox handles for use in BA213's. If you are looking for one keep your eyes open for DECserver 500 and 550's. The 500's use the 1/2 meg version and the 550's use the 1.5 meg. There is some difference in the firmware used on the DECserver CPU's from the usual PDP11/53 but I have never taken the time to play with them. I just put in the PDP11 firmware and run them. Dan