In article .com,
TheOneKEA writes
Let's consider a simple bit of track:
[...]
Thanks; I get it now. You should write another article for your site
that explains all of this.

One day.

If, say, there's a set of trailing points between 7 and 9, 7HR can
only pick up if:
- D is clear
- the signaller has set the route from 7 to 9
- the points are proven to be locked in the correct position.

With facing points, the HHR has to include logic to "look ahead" in
the correct direction.

So what does the circuit fiagram for this logic look like? Presumably
it involves the detector boxes on the point blades as well as linkages
to the HHR relays in the signals in advance of the facing points.

No.

As soon as you put points or other issues into the system, it changes
from simple automatic signals to far more complicated controlled
signals. [The following applies to NR, not LU, though LU is similar in
many respects.]

There is a list of something like 35 items that have to be satisfied
before a controlled signal may clear from red (not all apply to every
signal, of course). The relay logic to implement this is completely
different to automatics.

The logic will be defined in terms of a "route", which normally runs
from one signal to the next over a specific bit of track. Conventionally
the routes from signal 5 are called 5A, 5B, 5C, etc. with 5A being the
leftmost one. [You also get the distinction between 5A(M) 5A(W) 5A(C)
and 5A(S) - (M) is a normal running signal, (W) is a "warner" route with
a reduced overlap, so the signal can only clear to yellow once the train
is stopped at it or almost so, (C) is calling-on into an occupied
platform, and (S) is shunting).]

The signalman attempts to select a route. Each route has a set of
requirements, such as certain points being either in the right position
or available to move to that position. It may also be incompatible with
other routes. If the conditions are all met, however, a route relay
(e.g. 5ARR) picks up. This triggers the movements of points and locks
out conflicting routes.

Consider something like this:

|-O\ 5 101 |-O 7
----+------A-----+-B--*---------+------C-------+-----E--I---
\ |-O 9
\-------+------D-------+-----F--I---

Once the points are all in the right position and all the track circuits
in the route are clear, a second relay - 5GR - can pick up. This is
driven by a circuit something like:

+ve - 5ARR --- E TR --- C TR --- 101 NKR -*- B TR --- A TR --[5GR]-- -ve
|
+ve - 5BRR --- F TR --- D TR --- 101 RKR -+

X TR is up when track circuit X is clear. 101 NKR picks up if points 101
are detected in the locked position - that is, both blades and the bolt
are all correct. The NKR and RKR relays are driven directly from
microswitches within the points machine or related mechanism, checking
the exact position of the blade.

So 5GR indicates that the route is clear. This in turn drives 5HR, which
we discussed in a previous message, through various other tests such as
the junction indicator being correctly lit as appropriate. [Again, I
omit various details.] Finally, to answer your question, 5HHR (which
allows 5 to move to double yellow or green) will be driven as something
like
5HHR = (5ARR AND 7HR) OR (5BRR AND 9HR)
That is, the route relay will be used to decide which signal to look
ahead to, while the "signal can clear to yellow" logic is used to check
the points are set correctly.

====

Having written this, I've just remembered something. Look into Google
last year for posts in uk.railway concerning platform 4c (I think) at
Birmingham New Street.

If I recall correctly, in some installations using what's called
"geographic logic", the HHR logic looks more like:
5HHR = (101NKR AND 7HR) OR (101RKR AND 9HR)

The problem at Birmingham was that an extra platform was added via an
extra set of points, as if we modified the above:

|-O|\ 5 101 |-O 7
----+------A-----+-B--*---------+------C-------+-----E--I---
\ |-O 9
\--*----+------D-------+-----F--I---
102\
\--+------Z-------]

The extra logic for 5CRR was all done correctly, but nobody remembered
to include 102NKR in the 5HHR logic. As a result, 5 would show double
yellow (or green, I forget which) if the route was set into the bay
while 9 was not red. Oops.

--
Clive D.W. Feather | Home:
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