PDP 11/05 S vs 11/05 NC

[Noel Chiappa]

    > From: William Degnan

    > *why* did they make these two versions of the high-profile 11/05?
    > What was one used for vs. the other? *Why did DEC do this*?

Check the dates on the machines. I'm pretty sure the -NC is the earlier
version, with the bespoke H750 power supply (mounted alongside the boards, not
at the end of the box as with the BA11-K, the basis for the -S).

My guess as to why they upgraded from the -NC to the -S is that the latter
used the H765 (plus 'bricks') power supply system, in common with the BA11-K
and also the later PDP-11's (40's, 45's, etc). That probably provided a raft
of advantages, including i) greater flexibility in the current and voltages
being supplied, and ii) commonality with DEC's other machines (i.e. lower unit
cost -> great per machine profit margin). The BA11-K based -S also (by being a
couple of inches longer, and mounting the power supply off the back) had a
couple of inches more width for boards, enough to hold an extra quad system
unit. And the H765 may have been technically superior to the H750, too.

	Noel