PDP-11/70 Boards

Noel Chiappa jnc at mercury.lcs.mit.edu
Tue Nov 30 12:06:16 CST 2021

    > From: Ed Cross

    > I'm currently restoring a PDP-11/70 system and need the following
    > boards to complete the CPU: FP11-C

>From your mention of the FP11-C, I gather your -11/70 has a KB11-C (later)
CPU, not the KB11-B CPU of the earlier PDP-11/70's (prior to 1976 - the
difference between the two was whether they took the optional FP11-B or FP11-C

Not that it makes a big difference in your case; the 4 cache cards are the
same in both.

There used to be a seller on eBait (on the mid-East Coast - Baltimore, IIRC)
who was selling -11/70 CPU cards (I bought a whole spare set from him) but
alas he seems to have gone away (or sold them all; a quick search, both on
eBait, and in my email, didn't turm him up; I can institute a deeper search
if need be).

>From the blog of someone who got a KB11-A working, you'll really need KM11
cards; dunno if Guy Steele still has those clones he was selling.

There are definitely some versions of Unix which will run fine on -11/70's
without the FP11 (e.g. V6). The system binary is different for the
with/without versions, though: in the assembler code which saves the state of
one process before switching to another, there is code like:

        stfps   (r1)+

which will probably get an illegal instruction trap in kernel mode on a
machine with no FP11, and is therefore conditionally assembled (depending on
if the particular machine the system is being built for has an FP11). Perhaps
the later BSD versions look for the FP11 on startup, and adjust their
behaviour appropriately, but I'm not familiar with them.

V6 as distributed contains system binary for an -11/40, which will run on
_any_ -11 UNIX will run on, and can be used to build appropriate system

(Diversion: I've never found out whether the KB11-B and KB11-C of the -11/70
used/could use the same backplane or not. By examining the prints for the
boards of the FP11-B and FP11-C, and seeing on which pins they exchanged
signals, and what signals they exchanged with the rest of the CPU, and on
which pins, it should be possible to work it out. Ditto for the M8133 ROM and
ROM Control of the KB11-B, replaced with the M8123 in the KB11-C.)

(Interesting factoid: the M8123 is the only card shared between any variant
of the -11/45 and -11/70: both the KB11-C and KB11-D use it. Of course, I
think we're still missing a wirelist for the -11/70 backplane, of any
variant; and the ECO history. There appears to have been at least one poorly
documented upgrade; see here:


for more.)


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