On 29 Jun, 2007, at 20:37, cctalk-request at classiccmp.org
From: "Billy Pettit" <Billy.Pettit at
Billy replies: This was a serial machine using a magnetic drum for
So the registers and memory were all bit serial and on the drum. I/
accomplish by moving data from one line to another. The logic at
of the line was in peripheral but the bits were on the drum. So it
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
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
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).
Billy: I salute you. I wish more people on this list had your
love of old metal and were restoring it. I enjoy hearing about your
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
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.
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
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!
And many of them were wonderfully different and
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
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.