TODO(kurt): Find out whether the RIGHT plug-in slot is special in 11400 and DSA600 scopes. It seems likely that it is, given that the 11T5H manual says (section 1-1):
The 11T5H is used with Tektronix Digitizing Mainframes.
and lists only 11400 and DS600 mainframes in the mainframe compatibility table (table 4-4), and the same document (section 1-7) says
Note: If the 11T5H is to be used to trigger on signals originating from an 11A34V Video Amplifier, the 11T5H must be installed in the rightmost plug-in compartment of your mainframe.
So it seems likely that in 11400 and DSA600 scopes, the trigger signals are routed to the RIGHT plug-in slot, same as 11300 mainframes. However, unlike 11300 mainframes, 11400 and DSA600 mainframes allow the RIGHT plug-in to function as a vertical plug-in, i.e., its main signal output (pins 11A/11B) can be used as the vertical axis.
Gregor: I am not an expert in 11000 series scopes at all, but some Tek documents (like the 11000 series interface description) and the existence of the 11k detector on B20 to me are a strong hint that the 11000 series scopes was initially designed to be compatible with the 7k plugins, but this feature was dropped later in the design stage? It would be interesting to mention if that is correct. Otherwise the B20 pin doesn't seem to make any sense.