I have a VAXstation 3100 M38 which I have recently set up. All I have is the base unit and an expansion storage unit, I do not have the monitor, keyboard or mouse. In the first instance I had the machine connected to my PC. To do this I had to put the S3 (Alternate Console Switch) in the UP position and then connect the printer port to a serial port on my PC using a DECconnect cable and an adapter to connect it to my PC’s serial port. This makes the machine into a VAXserver rather than a VAXstation. With this in place I was then able to use HyperTerminal to get to the machine’s console, install VMS etc.
After setting up the machine in this way I was fortunate to be able to acquire a real VT220, with a keyboard. This meant that the machine no longer needed to be near my PC in order for it to work and I was able to move it to a more convenient location, and I now access it using Telnet.
The problem is that if you switch off the terminal, it sends a break signal to the serial port, which causes the machine to go back to the boot console. This means that the terminal needs to be left switched on in order to keep the machine operational. I don’t want to do this because I don’t want to use unnecessary power and I don’t want to risk any screen burn. The VT220 does not use that much power and I could turn down the brightness, but I still would prefer not to need the terminal and make the machine headless.
It turns out that there are two ways to achieve this.
The first method is to leave the alternate console switch in the UP position, but disconnect the DECconnect cable from the terminal before switching off the terminal.
The second option is to put the alternate console switch in the DOWN position (which is the default) and connect the VT220’s keyboard to the VAXstation keyboard port. This makes the machine think it is a VAXstation (rather than a VAXserver) and that a display is attached. The machine booted but of course I could not see the console (I did not try connecting the display only to the printer port to see what happens in that case).
Both methods work, but I think I prefer the former because it means that I can see the console if I have to.
My thanks to the people on the comp.sys.dec newsgroup for helping me out with much of the information that is in this posting.
Rob's Old Computers 
If I’m remembering right, VT220 used 25 pin RS-232 for serial connection. DTR was high when the VT220 was switched on. This signal was required for the VAX to operate properly, and it sensed a lack of DTR as a break signal. The ‘trick’ I remember was to cross connect on the VAX end pins 20 and one of the other pins (forgotten which) which were high. An RS232 break-out box would indicate which. Some DEC computers (like the PDT11-150) didn’t raise any pin high until it sensed a DTR high. So that trick couldn’t be done. We got around that by a bit of internal modification so that one of the computer’s RS-232 pins (think it was pin 21) was fed directly with +12v so had a high from which to connect to the other pin – so the computer always thought there was a terminal connected – even when there wasn’t..