"Paul Weaver" wrote in message
roups.com...
On 13 Mar, 22:29, "Peter Corser" wrote:
The half minute timing accuracy was down to the programme (sequence)
machines stepping every half minute. The timetables on the machines ran
in
half-minute time (from 0300 to 0300 the next day) with 0 at midday - half
minute time was the most you could do within the limits of a computer
integer (8 bit - +/-32767 IIRC).

8 bit would give you +-127, 16 bit is needed for 32767

The Central Line computer control ran internally to quarter minute
timings,
but the Timetable software used in developing and printing of the
published
timetables (and also the computer control timetables) could only cope
with
half minute resolution.

From noon until 3AM needs a signed integer capable of storing upto
1800 values for a half minute resolution, 3600 for quarter minute. 12
bits would do -2048 to +2047, capable of half minute, but not third or
quarter. 12 bits is 3, 4 bit words.

Nowadays of course 64bit time_t is the way to go, although I think
some libraries do 128 bit, which is a little extreme, although some
may say it doesn't go far enough. I think* a 256 bit time_t would be
capable of representing any measurable point in time, and then some.

*
seconds in creation (50 billion years): -- (86400*365.25*50000000000)
Measurable Units of time (plank time) in a second -- 1/(3.3 x 10^-44)
Measurable Units of time in creation (a*b)
~ 4.8 * 10^61
ln(4.8 * 10^61)/ln(2) == 205

Paul

Thanx for corrections - my memory was slightly befuddled & confused! It's a
long time since I did programme machine design for the original Heathrow
extension!

Programme machines (correctly known as sequence machines in most cases)
stepped in 30 second granularity. The whole system ran in two second time.
It is the two second time which requires the 16-bit width.

Bearing in mind that this system was designed in a railway signalling
pre-computer age using some relays and an individually designed set of
controls accomplished with electronic cards reducing the bit width was
always a necessity. The sequence machines ran on the plastic pianola roll
with mechanical fingers principle.

As an aside you may be aware that the train running numbers (set numbers)
shown in the time table were the direct digital equivalent of a hex number -
477 digital was stored as 477 hex and 477 was the highest number normally
used (there was also nothing between x78 and x99) which also saved bits (ot
sequence machine fingers).

Peter
--
Peter & Elizabeth Corser
Leighton Buzzard, UK



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