Billy Pettit real disappointment

Roger Holmes roger.holmes at
Sat Jun 30 19:06:55 CDT 2007

On 29 Jun, 2007, at 20:37, cctalk-request at wrote:

> From: "Billy Pettit" <Billy.Pettit at>

> -----------------
> Billy replies:  This was a serial machine using a magnetic drum for  
> memory.
> So the registers and memory were all bit serial and on the drum.  I/ 
> O was
> accomplish by moving data from one line to another.  The logic at  
> each end
> of the line was in peripheral but the bits were on the drum.  So it  
> really
> doesn't fit the standard definition of memory mapped I/O, though  
> that is the
> closest description.  There were no channels per se.
> Al has just posted most of the G-15 manuals.  Have a look.  It  
> certainly
> qualifies for the weirdest machine I ever worked on.

Yes that's reasonably weird. I particularly like the weirdness of  
drum where the data spins faster than the drum and it stores even  
faster rotating data on the same band. Makes my 1301's drums with one  
read/write head on each bit track seem boring, though the tracks are  
grouped in fours as it is a 4 bit parallel by 12 digit serial  

The 1301 has a neat trick for its day, to transfer an entire band  
(four tracks, 200 x 48 bit words), it did not wait for the start of  
the track to come round, it started at the next decade (ten word)  
boundary to come around, worked up to the end of the band, set its  
core address back to the original start address (which had to be a  
multiple of 200) and worked up to the start of the data already  

> ----------------
> Roger Holmes:
> Remember? I am restoring/maintaining an ICT 1301 which has individual
> Germanium transistors, wire-OR, four and gates to a PCB, one flip-
> flop one a PCB, a clock derived from the timing track of the last
> addressed drum store, a core store unit weighing half a ton an stores
> just 2000 x 48 bit words (plus 2000 x 2 parity bits). Its got Ampex
> TM4 mag tape drives (not industry standard 7 or 9 track, these are
> ten track units with hubs the same design as professional audio tapes
> and the 2 and 3 inch wide video tapes once used by TV broadcasters).
> [Snip]
> ---------------------
> Billy:  I salute you.  I wish more people on this list had your  
> energy and
> love of old metal and were restoring it.  I enjoy hearing about your
> efforts.

Thank you very much. The plan is to get one of my two machines into a  
museum one day, hopefully both when I am too old to use it/dead.  
Almost ten years ago the Computer Conservation Society had plans to  
get one into the Bletchley Park museum and one into the 'Science  
Museum' but both of them dropped out - I suspect its because a 1301  
takes up too much floor space.

> I'm also working on a germanium transistor wired-AND core memory  
> machine.

Ah! Wire-AND. Would this be negative power rail germanium by any  
chance? If so I suspect the circuits are basically similar but the  
nomenclature is different. On the 1301, a logic 1 is -6.3 volts and a  
logic 0 is 0 volts. When fault finding we usually have one person at  
the console operating the machine and looking at the logic diagrams  
calling the circuit references to be checked and one on an  
oscilloscope out of sight describing the display on the 'scope. Most  
of us are of course familiar with silicon logic and we get confused  
by calling out 'high' and 'low' because it is so natural, and we find  
it so difficult to remember high is 0 and low is 1, so we TRY to call  
out logic 0 and 1 instead, but it so hard to break old habits.

> It used TM2's not TM4's. And I've been unable to find any tape units.

The 1300s had four different tape subsystem options. The 'High  
Speed' (90kc/s) system, called tape type 1 (because it was the first  
developed) was a one inch system using Ampex drives, and IIRC, they  
were TM2s. They had 16 tracks (8 data and 8 CRC), and ran at 150 ips.  
My machines TM4 system (4 data 6 CRC tracks) was tape type 3 (22.5kc/ 
s) and was basically the same electronics with some parts removed and  
the minimum changes to make it store two frames where the high speed  
system saved one frame. It runs at only 75 ips. It uses Thyrotrons to  
turn the pinch rollers on and off. It has small vacuum chambers plus  
swing arms to buffer and measure the amount of buffered tape, the  
(air damped) sensors on the swing arms being the only things which  
control the reel motors - too much slack and the reel motor winds  
some in, too little and it winds some out.

>   Did
> find the original card reader, a modified Burroughs.
> Still, I consider what you are doing to be the true goal of classic
> computers.  I read, enjoy and participate with the microprocessor  
> based list
> threads.

Me too, though much of it is too much like my everyday job. Its like  
in the winter I drive my reasonably modern Jag or BMW to work but in  
the summer I enjoy driving my 1960s Daimler or Rover, and sometimes  
it feels like an achievement just to get to work, and if I stop for  
petrol I have to allow an extra ten minutes to chat to people who  
come over to admire the car. "They don't make them like that any  
more" or "My uncle used to have one of those". I even had someone who  
seemed to offer me 30k for a car insured for only 11.5k.

>   But my real love is in truly "classic" computers like yours.

Good. I was lucky enough to buy not just the computers but a load of  
spares. As yet I've not needed to do much component level repairs.  
Its the Ampex decks which cause most problems because they are  
American made, they don't use British components. For the CPU things  
like light bulbs, the same type were telephone exchanges and cars of  
the period, which are now classic and hence there is an industry  
still supplying them. The telephone exchange bulbs crop up on eBay,  
even sometimes the rarer voltages like 17 volts, and I recently  
bought almost a hundred 28 volt ones which produce an identical  
intensity plugged in in place of 24 volt ones. Its the things like  
the bulbs for the virtual address display (12 miniature projectors,  
one for each digit 1-8, and letters 'E','L' and 'U' and an  
underline)  which are a problem. They are 6.3 volt 6 watt. I thought  
I'd cracked it when I picked up some 6 volt car ones rated at 6  
watts, assuming that like a 12 volt car system, the charging circuit  
would run them up to 15 to 20% over-voltage. But they only last about  
30 seconds. I've checked the supply and it is only 6.3 volts, I can  
only assume it must be the enclosed space require a higher operating  
temperature. Googling has found the GE part number listed as  
microscope illumination lamps, but it doesn't give the voltage or  
amperage ratings, and the price was horrendous, so I'm not even sure  
they really are the right ones, and too tight fisted to take a  
chance. Maybe if we have a lot of people paying admittance to see the  
old girl (if she's works on the day) at classic car show I hold at my  
home in a couple of weeks time. Still, I've bought an old 4 trace HP  
storage scope (via ebay) on that expected income already, and after  
only one day's operation it now works for 20 seconds after turning on  
and the goes clip-clop, clop-clip, repeatedly and so I may have to  
spend more money and time trying to repair it. Better go to bed now,  
its 1am, got carried away!

> ----------------
> Roger Holmes:
>> And many of them were wonderfully different and creative.
> Indeed. And some of them almost make you cry because so much more
> could have been done with the same amount of electonics. My machine
> has been modified to implement an index instruction. Previously all
> indexing and indirection had to be done by program modification, and
> even now subroutine return is done that way (see my previous e-mail).
> I have one machine in 'conserved' state, unmolested, unrepaired non-
> runner, and one with extra tweeks and darn right mass rewiring which
> runs and I can't stop thinking about how it could be improved, yet
> somehow manage to stop myself doing so. There are so many gaps in the
> instruction code and spare bits in the instructions etc. The only
> modification I am working on plugs into an extension port lashed up
> by a previous owner. This is to capture the data from the machine
> onto modern media. May replace with an RS232 interface later to drive
> a teletype and/or pen plotter, and/or a parallel inteface for a
> Friden Flexowriter.
> [sni]
> Roger Holmes.
> Classic computer collector, classic car collector, machine tool
> collector/user (for the prior mentioned hobbies), and for a job,
> programmer of CAD and graphic software and printer/plotter drivers
> for Apple computers.

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