In message , at 10:12:00 on
Wed, 10 Sep 2014, Paul Corfield remarked:
And with Oyster there's at least the chance that you can see your
balance disappearing, if you know exactly where to look on a gate for
the fraction of a second it displays the number. I'm quite sure there's
no such facility for the contactless cards.

I'm a member of the contactless pilot and on my journey home this
evening both gates gave me red lights while opening but the exit gate
also displayed my fare. I've just checked the online version and that
is matching what I expected to pay and what was shown on the gate.

I'm not really sure how that happened. I'll have to look more closely
at the displays next time and perhaps try and get a picture.

Hmmm interesting. I wonder if TfL have responded to the trial
feedback about people not liking the absence of the fare to the paid
being shown on the exit gate display.

Once you take into account any OSIs that might be in play for the
current journey, then it becomes more complicated than just a case of
"where did you last touch in, and here's the single fare from there to
here".

It suggests (I'll put it no more strongly than that) that something is
written to the bank card.

I've been corresponding today with an acquaintance who is very much into
a range of Card technology (and is a 'Member of ITSO' - whatever that
implies) and he assures me that nothing can be written to a CPC
(Contactless Payment Card).

On the other hand he says that ITSO cards *are* designed to be read, and
written back to, whenever they are 'touched', to do things like mark a
ticket it's carrying as 'active' or 'expired'.

I cannot see how else an exit gate could calculate and display a fare
within the few hundred millisecond processing time parameter. I can't
see there being contact with the "back room" system in that time
parameter.

And a displayed fare for just "the current journey" won't alert you to
issues with unresolved journeys earlier in the day, so perhaps it should
really be your "fare for everything so far today", but that would
require the back-office to run its reconciliation/capping program every
time someone touches out. And how would it deal with bus trips where the
"back room" won't have that information yet.
--
Roland Perry