Strange third party board in PDP-11/45

Mattis Lind mattislind at
Sun Jul 22 12:25:14 CDT 2018

söndag 22 juli 2018 skrev Paul Birkel <pbirkel at>:

> 26 bits (or 13 bits) doesn't make any sense on a 16-bit machine; makes
> more sense as a high-speed I/O buffer.

One can note that it is actually two different types of 1k chips. 16 chips
are 94L415 and 10 chips are 93415. As far as I understand the L is the
slower variant.

This could mean that 16 bit data is in the L chips while the faster chips
are used for a 10 bit cache tag. Maybe 8 address bits plus some valid bit
and possibly a dirty bit?

The switch is marked ON/OFF which could simply cache on/off. The
handwritten label on the board says that it is not in use and should sit in
slot 21.

And of course those two I/O connectors don't belong on a cache.

Those IO connectors are connected to two double height boards in 26 /27 AB.
They are also made by ACT and contain a few TTL chips.

So it pulls out some signals out of both Unibuses but 20 + 10 signals at
most it not much of a complete bus so I wonder what kind of signals go

> While odd to use slot 21 (Fastbus) for something other than memory I don't
> know why a fast memory-mapped I/O channel couldn't go there.
> Also note all of the signals employed on tabs C-D-E-F?
> It may not even employ the Fastbus; just talk to Unibus B.
> Unfortunately there's not much documentation for the MS11.
> It seems likely that A-B isn't anything like the usual Unibus signals, and
> who knows where the Fastbus signals are routed.
> On my backplane D-E are essentially unused whereas A-B-F are busiest.
> I see the marking "copyrighted 1976", which is rather earlier than the ACT
> / ABLE documentation online.
> From Bitsavers see the ABLE documents for the SCAT/45:
>     Able_Computer_Product_Summary.pdf - page 3
>     Able_Computer_Product_Brochures_1982.pdf - pages 16-17
> The PN 10003 doesn't seem to match anything documented from ACT, however
> it's consistent with them.
> The original QBus Univerter is PN 10001, and is dated 1976.

There are some documentation to get with the machine so the manual for the
board might turn up.

> What are the pair of DIP24 ICs on the lower-left?

Fairchild 9308 Dual 4 bit latches.

> -----Original Message-----
> From: cctalk [mailto:cctalk-bounces at] On Behalf Of Paul
> Anderson via cctalk
> Sent: Saturday, July 21, 2018 6:54 PM
> To: Mattis Lind; General Discussion: On-Topic and Off-Topic Posts
> Subject: Re: Strange third party board in PDP-11/45
> I think it's Applied Computer  Technologies, and I think they made cache
> and several other options. They were popular back in the day. I have a
> bunch of their boards here.
> Paul
> On Sat, Jul 21, 2018 at 1:37 PM, Mattis Lind via cctalk <
> cctalk at> wrote:
> > This board was sitting in slot 21 of the backplane in a 11/45
> >
> >
> >
> > What kind of board is this?
> >
> > It has 26 bipolar RAMS. Fairchild 93415 1kbit SRAM.
> >
> > The manufacturer might be ACT whatever that is.
> >
> > My guess is that it is some kind of cache board? It is connected to both
> > unibuses in the machine.
> >
> > Better ideas? Documentation?
> >
> > /Mattis
> >

More information about the cctech mailing list