> On Mar 13, 2025, at 4:35 PM, ben via cctalk <cctalk@classiccmp.org> wrote:
>
> On 2025-03-13 1:36 p.m., Paul Koning via cctalk wrote:
>
>> Depends on which one. RTL was 3.6 volts positive, as far as I can remember.
>> I actually have a keyboard that has some of those devices in it. Yes, ECL
>> is around 3 volts also but negative supply. And of course some people
>> designed systems with positive supplies but "negative" logic, in the sense
>> that ~0 volts is logic 1 while near-VCC is logic 0; the CDC 6000 series
>> machines are an example.
>
> I was thinking of the PDP-8 there. 0 Volts logic 1 -3 volts logic 0.
>
>> FPGAs come in amazing sizes if you have sufficient money. I hope some day
>> to cram an entire CDC 6600 into an FPGA. The main problem with this isn't
>> FPGA sizes (by today's standards, an upper-midrange FPGA can do the job,
>> memory included) but rather the creation of an accurate model given the
>> bizarre and hairy timing of that machine. I have a gate level model, but it
>> doesn't work yet because of those issues.
>
> What would the purpose of said computer be?
> Might be better off with a clean 64 bit design and 16 bit bitslices.
To understand fully how the 6000 machines work, and to run code for those
machines more accurately than can be done on emulators. Also "because it can
be done" -- the same reason a lot of us do most of what we talk about on this
list.
A while ago I used the VHDL model to understand a fairly well known but totally
undocumented detail of debugging peripheral processor programs from memory
dumps (the only tool available if the program gets stuck or lost). That
detail: a system reboot will drop a word of zero into each PP memory, usually
at the place its program pointer pointed at the time the boot reset happened.
The data flow through memory, the processor guts, and back to memory explains
why, but you have to look really closely at low level block diagram
documentation to see it. On the other hand, it appears quite plainly when you
run the process on the VHDL model in a simulator.
paul
