Kevin, what you are basically talking about here then is how do diagnose and debug a broken serial connection. It's the same no matter if you are running SLIP, PPP or DDCMP on that serial link. It's still just a cable with GND, RX and TX, and possibly some modem control signals. And it is no different if you instead are doing interactive traffic, such as a normal terminal connected to a computer. It's still the same serial connection. If you want to understand how a serial port works, that is excellent. But the first thing you need to understand is to differentiate between the physical layer, and the communications protocol that runs on top of that physical layer. As for PPP or SLIP on VAX/VMS, no, that did not exist earlier than DECnet over serial, as VAX/VMS had DECnet before it had TCP/IP, and SLIP can only talk TCP/IP. PPP can in theory run any protocol, but it was still not implemented on VAXen for DECnet. But my point was that if you want to tunnel DDCMP, it will be exactly the same solution you'd do if you wanted to tunnel any protocol using a serial interface. There are absolutely no difference from your tunnel point of view whatever protocol is used on the link. It will only be a stream of bytes on a serial link anyway. And yes, it will work just fine running DDCMP on top, and you can get DECnet connectivity that way. However, it is not very fun, since the actual speed of the serial interface will be 9600 bps no matter what, if you have physical machines involved, since DECnet insists on setting the speed of DDCMP interfaces to that, or slower. Johnny On 2012-01-03 04.02, Kevin Reynolds wrote: Johnny, First and foremost, I want to learn how to configure and understand how these devices connected historically so I can troubleshoot the connectivity. This gives me the best feeling of true understanding. Unfortunately, I don't have a slew of these systems (I have a single Microvax II) that I can use to interconnect, so I am locked into simulating at least one end of the connection. I was hoping to do this with simh. Secondly, I was hoping to create error conditions and determine whether I can diagnose the problem. As this is legacy hardware, protocols, and otherwise, I understand that aside from understanding how some of our modern protocols and systems emerged from knowledge of the old systems. I am I really hoping to start a little further back to solidify my understanding of modern technology without all of the additional layers of distraction included with modern systems. Did the VAX/VMS platform have an option to include PPP or SLIP previous to decnet +? I have a good knowledge of these protocols. I used slip back in the day when I had to connect with a pr1mos system that wouldn't allow pass-through connection to the internet. PPP, especially multilink using PPP I have implemented multiple times (even recently...silly, but I didn't choose). My ultimate goal is to pass decnet phase IV (not plus) traffic over the connection. Thanks, Kevin ------------------------------------------------------------------------ link. support