Advice needed: Entry point into things PDP-8

[Ethan Dicks]

On Mon, Oct 22, 2018 at 10:39 PM Carlos E Murillo-Sanchez via cctalk
<cctalk at classiccmp.org> wrote:
> you.  The thing
> is, I would like to have something pdp8-ish that would allow me to play
> a little bit
> with the programming languages that were available for these machines,
> FORTRAN 4K and
> FORTRAN IV in particular.  Now,  I would love to be able to time some
> FORTRAN jobs just
> to get an idea about what it was like back then.  I am aware of PiDP-8,
> simh, as well as
> SBC6120, SBC6120RBC.

I would probably do all the things but in a particular order.

If my goal was to learn PDP-8 software, I would just start with simh
running on anything.  I have a PiDP-8.  It's nice.  You definitely get
the feel of running an older PDP-8 (except no noise for floppy drives
or DECtape, and no seek time) but under the blinky covers, it's
running simh.  You can learn everything about the configuration of
PDP-8 models, about memory, and all the programming languages with
simh.  From there, consider a PiDP-8 if you want a quick junior-sized
emulated machine for the look and feel of things.

The SBC6120 with FB6120 is also nice.  I have one.  Unlike the PiDP-8,
the SBC6120 has a native 12-bit microprocessor (IM6120) and isn't
emulating anything except running some front panel ROM code to handle
disks in a way that hides the details from the OS/8 side of things
(much like the DECmates do).  Unfortunately, they are expensive.  Not
many were made and they weren't exactly cheap when new.  As for the
GAL programmer, the GALs in the SBC6120 are rather ordinary but, yes,
some programmers can't handle them.  I have multiple device
programmers and even the one from 1990 can program 20 and 24-pin GALs
(I'm not sure about newer programmers - I don't have one because my
older ones work fine).  Someone on the list could probably bang them
out for you for a few dollars unless you really wanted to own the
entire hardware toolchain.

>From there, one of the challenges of repairing your VT78 and VT278
boards is there's no blinkenlights console to assess repair status
during the repair or to try to toggle in test programs.   Replacing a
ROM is easy enough, even if you have to make a pin-swapping socket
adapter to use a modern EPROM (I don't know what type of ROM is in the
VT78, but it's possible that it's something standard like a 2708 or
2716).

Getting replacement parts for your VT278 is probably not all _that_
hard - one likely source is a DECmate II or III board, if you can find
a spare.  Last time I bought one, it was about $50 but it probably has
gone up a little.

The VT278 does have a compatibility issue with the console SLU (03/04)
and the flags.  That's why there's OS/278 in the DECUS collection.  If
you were going to write your own code from scratch, you'd have to take
the operation of the console device into account when writing your TTY
code, and you might not be able to just run anything you find.  I have
a VT278 but have only run WPS-8 on it, not OS/278 so I can't provide
deep details.  You would likely want an RX02 (and cable) if you were
going to resurrect the VT278.  The cable can be made (DB25 one end,
DC37 other end, and the pinouts are available).  Inside the VT278
pedestal is a _real_ RX02 but there's a simple 40-pin-to-DB-25 passive
board inside the case, much like the one in an RX02 for a MINC-11,
and, again, this can be made new.  Finding any RX02 drive is an
exercise in shopping and paying freight, but then you'll need someone
to at least cut some starter floppies for you.

So if you are going to go the emulation route, I'd start with just
downloading simh and some OS/8 image files (RK05 or RX02) and learning
the software side of things.  This may generate enough interest to
want to explore the hardware side of things, and for that, you
definitely have a swath of projects in front of you with varying
degrees of cost and effort.

-ethan