Confirm this PDP-8?

From: Ethan Dicks <>
Date: Wed Oct 6 20:54:18 2004

On Wed, Oct 06, 2004 at 07:28:26PM -0400, Dan Veeneman wrote:
> This weekend I picked up some classic computing
> gear including a PDP-8. I believe it's an 8/A, but since
> my PDP experience is limited to the PDP-11 series,
> I'm not sure.

Yep... it's an -8/a...
> Can anyone take a look at
> and let me know if my assumption is correct?

What you have there all looks correct. You might want to post pictures of
the unmarked hex-height cards (see below for an educated guess).
> I haven't had time to even clean it off, but when I do
> have a chance I'd welcome guidance on checking
> things out step-by-step to get it back into
> operating condition.

Well... first off, you'll need a front panel. The circuit is trivial, so if
yours was lost in storage, you could make one out of a couple of toggle
switches, some LEDs, and maybe a chip or two (I can't remember what's on it
at the moment).

Second, I don't see an RX8E in there (M8357). The 40-pin cable sticking out
of the front probably went to it (presuming the other end is in the disk

I do see MOS RAM (M8417), but I can't tell from the handle what size. The
three varieties are MS8CAA (16kW), MS8CAB (32kW), and MS8DJ (128kW). Since
it doesn't appear that you have a KT8A (M8416), you probably have an MS8CAA
or MSC8AB. The way to tell for sure is to pull the board. If half of the
RAM spots are filled with 4K DRAMs, it has 16kW. If all of them are filled
with 4K DRAMs, it has 32kW. If all of them are filled with 16K DRAMs, it's
128kW. I have never tried extracting the 4K chips and replacing them with
16K chips, so I can't say for sure if that's the only difference, but it is
a telling difference (since I _have_ a KT8A, I've always wondered if I can
take one of my 16K boards, extract the 4K DRAMs and upgrade it with 4116s).

Of the other 3 boards, the standard compliment in a small-ish -8/a system
would be a KK8A CPU (M8315), a DKC8AA console I/O board (M8316), and
KM8AA bootstrap/memory extender (M8317). There are several varieties of
KM8AA, but in the field, I have only run into the fully-loaded ones (all
features installed/enabled).

There are, IIRC, three sets of boot PROMs for the KM8AA, but that would
only matter if you were trying to change your system device from the RX02s
(I have one set at home that will boot an RL01 on an RL8A, and two sets
that will not, for example). There's not much room in them, so if you
needed to boot, say, a TU56 vs an RK05 vs an RL01, you'd probably need
both of the later sets. I've only ever needed to boot floppy or RL01,
so in practice that's never been a problem for me.

Doug Jones' module list is a good place to start if you know the handle
numbers and need to translate...

The 1976 or 1977/78 'small computer handbook' will have lots of details
about jumper settings, periperal features, etc. IIRC, one or both are
on bitsavers (you can do with just the 1977/78 edition).

Getting a hard drive on that machine might prove to be a bit of a
problem. Essentially, your 'common' choices would be RK05/RK8E
or RL01/RL8A. I _think_ it's easier to find the RK8E than the RL8A,
but I'm pretty sure it's easier to find RL01s than RK05s at this
point. It's certainly easier to find RL01K (cartridges) than 16
sector RK05 packs, but that's because _all_ DEC machines that can
talk to the RL01 (PDP-8, PDP-11, VAX) use the same packs, whereas
the RK05 used differently hard-sectored packs for the PDP-11 and
the PDP-8.

There were 3rd party disk controllers for the OMNIBUS, but they are
substantially less common than either the RK8E or RL01. I know of
a (6809-based) SCSI controller, but I have never seen one in person.
It knew how to talk to both 5.25" floppies and MFM hard disks through
an old OMTI SCSI bridge card. It works with ZIP drives, etc., as
well, from what I was told ten years ago.

The good news is that RX01/RX02 floppies are durable and can still
be found here and there. I have several boxes at home, but won't
be in a position to do any duplicating for several months, at
least. It is possible for other varieties of hardware to duplicate
RX01-density floppies, but, unless someone has tricked out a
Catweasel, only and RX02 can write to an RX02 (mixed density tricks
specific to that drive). There was some discussion here some time
back about what it would take to hook the drive unit itself up to a
modern machine, but someone mentioned that the parallel port on a
PC-class box wasn't fast enough to feed the drive what it needed
when it needed it, and the thread died. If you _could_ hook the
drive up to a machine, it would be a lot easier to make and archive
disk images.

You have plenty of memory to run OS/8 (8K required, but more is
better), and if you happen to have 32K, you could run Adventure,
one of the larger programs produced for the PDP-8. The assemblers,
and FORTRAN compilers, BASIC interpreter, and FOCAL interpreter
are all fine with less than 32K. One thing nice about your rig
is that with extra serial ports, once you have an OS and Kermit,
it will be easy to stick new programs on the machine. You might
even be able to run a dual-head WPS-8 machine (with a second
floppy drive and controller).

There are _lots_ of PDP-8 resources on the 'net. More than one
excellent web page is maintained by members of this list.



Ethan Dicks, A-130-S      Current South Pole Weather at 07-Oct-2004 01:10 Z
South Pole Station
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Received on Wed Oct 06 2004 - 20:54:18 BST

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