PDP-8 on FPGA project & where is Hans Pufal?

Vincent Slyngstad vrs at msn.com
Thu Dec 18 22:37:48 CST 2008

From: "Philipp Hachtmann"
Sent: Thursday, December 18, 2008 11:08 AM
> Here the entity declaration of my teletype interface, just to give you an 
> imagination:

[good stuff elided]

> The break interface is not used by the TTY. I included it just for 
> completeness... The wires end open.
> And there's no CLF line yet....

You definitely need the "reset" that the CAF instruction (and the
"clear" key on an 8/e triggers).  For an 8/i, it's BINITIALIZE,
which is triggered by the "start" key on the front panel.

> If multiple IO devices are to be connected, the IN lines (to the CPU) must 
> be OR'ed together.

The original PDP-8's have these active low, and a logical OR is
implemented with wire-and, via open-collector drivers.

> When my project is in a usable state, I would like to put it on 
> opencores.org.

Cool!  It might be an interesting point to work from for what Henk
and I are trying to do.

>> We have Hans' PDP-8 code from a few years ago, and John Kent did some 
>> nice work a year or so ago with us to clean up the build and to port to 
>> the XESS FPGA card.
> VHDL or Verilog code?


>> I've got the code running FOCAL and other stuff in John's configuration, 
>> and the 8/i front panel is pretty much working in that configuration.
> How is it attached? Shift registers?
>> There is currently a regression that broke the IOTs that FOCAL needs, so 
>> the more cool code examples don't work at the moment.
> Hm... Which IOTs?

Most of them -- Skips are never skipping, and all IOTs clear AC.
Shouldn't be too hard to fix, but I haven't really looked into
it yet.

>> and (more importantly) because we don't feel we can redistribute his 
>> code,
> Sure, that he cannot be reached?

Right.  The word "license" doesn't appear in any of the files, and
we haven't been able to get a response from him on the subject.

>> I have recently started to implement a "direct coding" of the 8/i 
>> schematics in VHDL.  That will hopefully give a "clock accurate" 
>> implementation of the 8/i.
> What a task!!!
> If you try the real thing, you will get lots of angry warnings about flip 
> flop clock input abuse! DEC used the D-flip flops with preset and clear in 
> a (today) very uncommon way....!

I'm hoping to get around that by declaring TP1...TP4 (and *strobe*, etc.)
to be the clocks, and re-expressing the latches so that they are clocked
by an "official" clock.  I don't know yet how well that is working out,
as it's buried in with the other 188 warnings-of-the-moment.


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