The PDP-11/45 had split I/D capabilities.  It was not all that much
later than the 11/20 (2 years according to Wikipedia anyway).
On 7/13/2015 2:16 PM, Rich Alderson wrote:
  From: Kip Koon
 Sent: Sunday, July 12, 2015 10:52 PM
  I would be most interested in finding out more
about this effort.  Do you have
 ongoing pictures documenting this effort?  I'd love to have a PDP 8, 11, 12
 someday, but I don't have the space for something like that much less the cost
 involved so I'll have to be satisfied with emulators on my PC or eventually
 building one or more of these systems with current technology like the SBC6120
 if memory serves.  Are there other possible alternatives?  I used a PDP-8/E in
 high school and college and have been quite interested in the high capability
 PDPs like the PDP-11 Series for starters.  I didn't know there were PDP 12
 Series computers.  Are there other PDP series computers as well? 
 Don't confuse higher numbers with higher capabilities, or even as being related
 to each other.  DEC created computers with 9 different architectures before the
 32-bit VAX was even dreamed of.
 The list of Programmed Data Processors goes like this:
 PDP-1  18-bit word, 12-bit address
 PDP-2  24-bit word, paper design only
 PDP-3  36-bit word, paper design only
 PDP-4  18-bit word, 13-bit address
 PDP-5  12-bit word
 PDP-6  36-bit word, mainframe unrelated to PDP-3
 PDP-7  18-bit word, PDP-4 successor
 PDP-8  12-bit word, PDP-5 successor
 PDP-9  18-bit word, PDP-7 successor
 PDP-10 36-bit word, PDP-6 successor mainframe
 PDP-11 16-bit word, 16-bit address[2]
 PDP-12 12-bit word, PDP-8/i + LINC hybrid
 PDP-14 control processor for customer-built special purpose equipment
 PDP-15 18-bit word, PDP-9 successor
 PDP-16 Register-Transfer Module hard-wired processor, PDP-14 competitor[3]
 There were later variants of some of these:
 PDP-7A
 PDP-8/s, PDP-8/i & /l, PDP-8/e & /f & /m, PDP-8/A
 PDP-9/L
 PDP-15/76
 After 1971, they stopped naming things "PDP-n", with the exception of models of
 the PDP-11 (which eventually consisted of more than 20 models designated
 PDP-11/nn), but even there the Pro-3x0 desktop systems were called something
 else.  Later models were microprocessor-based.
 The later PDP-10 models were designated DECsystem-10 and DECSYSTEM-20.
 The later PDP-8 models were the DECmate, DECmate II, and DECmate III (word
 processing desktop systems) and the VT-78, all based on Intersil or Harris
 microprocessors which were roughly the PDP-8/e in silicon.
 "high capability PDPs" = PDP-10 & follow-ons.  PDP-11?  Pfeh.
                                                                 Rich
 [1] 1 built by a customer, reputed to be an NSA front company.
 [2] With memory management, 18 or 22, in 16-bit segments.  Late models could
     use separate instruction and data segments, for a total of 128KB in use at
     one time.
 [3] Different model lines had separate sales teams, and competed against each
     other across the company for sales.
 Rich Alderson
 Vintage Computing Sr. Systems Engineer
 Living Computer Museum
 2245 1st Avenue S
 Seattle, WA 98134
 mailto:RichA at 
LivingComputerMuseum.org
 http://www.LivingComputerMuseum.org/