Differences between the PDP-11/15 and PDP-11/20

[steven at malikoff.com]

A few months back you might recall the road trip I did to recover a Foxboro FOX-2, a rebadged
PDP-11 that ran the furnace oxygenation plant at the BHP steel mill at Port Kembla here in Oz
during the 1970s.

I had thought the machine was an 11/20 with a custom FOX front panel that (apart from the
Foxboro lime green colour trim) had only 16 address switches instead of the 18 found on that
machine, and that there might be two more vacant switch locations on the front panel PCB
to be found lurking underneath when it came time to disassembling.

But having just seen the PDP-11/15 up on eBay, I now believe that is what the FOX-2 really is.
This page (http://gunkies.org/wiki/PDP-11/20) reckons the /15 is just an OEM version of the
/20 but I'm wondering if there are any more differences apart from the lack of the two high
address switches? (Presumably the /15 could not run one of those KT-11 paging units as was
seen on that recent eBay 11/20)

Steve.