From: jl@glen--canyon.com
On Thu, 12 Feb 2026 17:36:12 +1100, Bill Sloman
wrote:
>On 12/02/2026 2:56 am, john larkin wrote:
>> On Wed, 11 Feb 2026 23:38:42 +1100, Bill Sloman
>> wrote:
>>
>>> On 11/02/2026 6:39 am, john larkin wrote:
>>>> On Tue, 10 Feb 2026 07:49:22 -0900, Christopher Howard
>>>> wrote:
>>>>
>>>>>> Standard apprach for low-accuracy approximation is diode-resistor
>>>>>> network. But such network gets inpractically large if you need
>>>>>> good approximation.
>>>>>
>>>>> Thank you. Looking at all the suggestions put forward, it seems like —
>>>>> for my application — that just getting the AD633 might be best choice.
>>>>> My analog computer is short one multiplier for a simulation I wanted to
>>>>> play with, and I wonder if there was some easy trick to get a reasonably
>>>>> accurate square function without getting a decent multiplier. In a lot
>>>>> of basic physics simulations, one has to square velocity to get
>>>>> things like drag or lift forces.
>>>>>
>>>>> Line approximation with a resistor-diode has some appeal. Maybe it could
>>>>> be accurate enough for my little educational experiments? I think, to
>>>>> pull it off, I would need to use trim pots — two per segment, for the
>>>>> biasing and the attenuating.
>>>>>
>>>>> The schematic from the neurological paper seemed to be a line
>>>>> approximation solution with some of the diodes in the op amp feedback. I
>>>>> didn't try to get the whole research paper so I'm not sure how one would
>>>>> work out the correct resistor values.
>>>>>
>>>>> I downloaded the information on the MC1496-D and LM13700. I see the
>>>>> basic idea is that these chips also produce products of signals, but
>>>>> beyond that I can't make any intelligent comments at present. My analog
>>>>> computer is usually dealing with signals in the range of a few hundred
>>>>> Hz up to a few kHz. I am curious if maybe something could be done with
>>>>> the gain control pin on the LM386 chip (I have quite a few of those). It
>>>>> looks like, using a series RC network, that the gain can be set anywhere
>>>> >from 20 to 200.
>>>>
>>>> A simple opamp circuit with some resistors and diodes can have a
>>>> several-segment approximation to a square root. I did that in a
>>>> steamship throttle control once and it worked well enough.
>>>
>>> Didn't you have a DEC digital processor to play with for that gig?
>>>
>>> The DEC PDP-8 I got to play with for my Ph.D. project could be persuaded
>>> to do multiplication and division - I used it for signal averaging - and
>>> square root extraction shouldn't have been difficult.
>>
>> Yes. I had a PDP-8 12-bit machine running FOCAL, and simulated the
>> steam valve, the turbines, the prop, the hull, and the ocean. I
>> plotted the responses on a teletype machine. We showed it to the
>> shipyard and the owners and they liked it and we got tho sell them the
>> engine room consoles. That was for the 32,000 hp, LASH ships. I think
>> I was a sophmore at Tulane at the time.
>Focal wasn't up to much. I programmed my machine in assembler
> - Macro-8 - and used interrupts to get tolerably precise timing.
Focal-8 was fabulous, a nice recursive language with floats and math
functions that ran useful stuff on a 4K-word 12-bit machine. It used
all sorts of hashing tricks to save core.
The internal stack-oriented architecture of Focal-8 inspired the cool
PDP-11 architecture which in turn inspired the 68K. Pity that IBM
picked the Intel dog and Microsoft.
Focal-11 was even better. But it had a terrible random-number
function, the silly modulo thing. I patched it to use a PRBS thing,
Rick Merrill liked it, and he sent me the program listing.
We keypunched it and spun our own versions. We did pipeline control
systems with that and wrote a version for the first satellite
navigation experiments that led to GPS.
John Larkin
Highland Tech Glen Canyon Design Center
Lunatic Fringe Electronics
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