On Sat, 12 Mar 2011 18:20:00 -0800 (PST), Jack
wrote:

On Mar 13, 2:07?am, john b wrote:
On Mar 13, 7:06?am, Jack wrote:
On Mar 12, 12:45?pm, john b wrote:

[...] If the light's gone green and the barrier's opened, I
don't think it's *possible* for the system to leave your journey
unresolved.

John - ?it is perfectly possible. ?The gate / validator, ?station
computer and hence the central system record the exit (or entry -
equally possible) but the exit (or entry) is not written to the card.
It happens from time to time.

John - the system adjusts data on the card chip during transactions.
The balance and / or season tickets are held on the card chip, not in
the central system. [...] There will always be a minority of transactions that are
not completely successful.
The point is that maximum fares are deducted during the transaction
with the gate or validator and not after the fact.

I would expect it to be designed such that the green light is not
given until the card has confirmed that it has successfully received
the update. That way, the transaction can be attempted again.

One of my old Oyster cards used to work on the Underground but on the
bus, 50% of the time a "Card Communication Failure" would happen
immediately afterwards. I wonder why that was... Of course failing
to touch in and out isn't a problem on the bus but as far as I know I
always paid the fa this wasn't a case of a "green light" followed
by an inability to debit the money, which seems to be the situation
discussed above.

Richard.