cctalk Digest, Vol 31, Issue 7

mark at mark at
Sun Jan 8 00:07:47 CST 2017

Allison wrote;

>I envy the chance to restore a LGP-30 or for that fact play with one.
>Many of the things I remember
>mid sixties on are now gone or were rare then.  Like small desk sized drum 
>computers using transistors or first generation IC (RTL and RDTL).

Rick Bensene wrote:

>I so regret not having rescued an old computer that I played with through 
>all four years of high school.
>The machine was made by Minnesota Mining and Manufacturing (aka 3M 
>Corporation). Today, there seems to be no record that 3M ever was in the 
>computer business. was.

I never saw one of those, but the computer center I worked at in college 
dumped a Cincinnati Milachron small business computer system - *not* a 
machine tool controller.  I tried to grab it, but that was not to be.  Same 
thing - there's no record anywhere I could find that they were ever in that 

> along with another old machine that was donated to the school...hardly a 
> computer,  more like an accounting machine, made by SCM, called a 7816 
> Typetronic.

I actually ended up with a complete SCM 7816 system, including:

- The I/O Printer, which was a hacked SCM electric typewriter, with 
diode-matrix encoding for the keyboard, and relay decoding for the printer; 
it used a row of washers to mechanically ensure that only one key could be 
pressed at once.  It also had a paper tape reader built into the back of the 
carriage, so that some computation could be triggered by the carriage 
position, or performed while the carriage was returning.

- The main 2816 control unit, with a plug-board "output panel" to route data 
between the various peripherals; this also had the massive power supply, 
which used a ferro-resonant transformer to regulate all of the voltages.

- The optional (!) 7816 arithmetic processor, which did bit-serial addition, 
subtraction, or multiplication (no division, but this was simulated by using 
reciprocal multiplication); there were nine 10-digit registers (no other 
working memory), all implemented on a fixed-head disk, plus a buffer 
implemented electronically.  Add time was 17ms, multiplication 700ms; this 
is why the ability to do calculations during the carriage return was 

- Two paper tape punches - these were re-branded CDC punches, and were very 
nice units.  40 characters per second, with a built-in automatic 
verification; they could also be used to punch on the side of cards, instead 
of tape.

- Two paper tape readers.  These were built by SCM, and were pretty nice 
too; they were optical, would read at 30 characters per second, and could 
stop from full speed on the next character.

- The custom desks, which included a recess for the I/O Printer to sit in, 
and acted as chasses for the 2816 and 7816.

- All of the manuals and schematics for the whole thing.  Some of the logic 
was made using thick-film modules, but most was on the vintage single-sided 
boards, with obviously hand-drawn traces and jumpers on the component side. 
Somebody has uploaded some of the manuals and 2816 schematics to bitsavers, 
but not the schematics for the 7816.

With the complete schematics, I was eventually able to get the thing to type 
and read and punch tape, but I never got the arithmetic unit working.  That 
machine was *really* dumb...  I carted the whole thing around for about 15 
years, until the new wife decided that she was more important than the space 
it consumed.  That's OK, I guess - I eventually ended up dumping her, too...
Mark Moulding

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