JMRI: Turnout Feedback
Model railroaders want different things when it comes to knowing the status of the turnouts
on their layout. Some are quite happy to say "I told it to move, that's good enough for me".
These people don't worry about whether a turnout on their layout actually moved when they
told it to. Those people can just ignore this page, and leave the JMRI settings at their
default.
But some modelers want to have better information about the status of the turnouts on
their layout, and so use some form of "feedback" to indicate turnout position. This could be
as complicated as two microswitches, adjusted to only close when the turnout is properly
seated in either position. Or it could be something simpler.
In the code, Turnout objects actually know about two different states: "Commanded" state
and "Known" state. The commanded state is "This is what was asked for last". The known state
is "This is what's actually correct on the layout as far as I know".
Built into into LocoNet and XPressNet are limited abilities to monitor what is happening
on their networks and adapt to messages about Turnout state changes. The NCE Powerhouse Pro
(only) can monitor the last known state of any turnout that it changed (on request from a
cab, macro, JMRI or Mini Panel). All the other protocols normally use a "you told it to
change, so it did" logic. In other words, when something in the code (the Turnout Tool or a
script) commands a state change from closed to thrown, the default is for both both the
commanded state and the known state to change to "thrown".
But it is also possible to control this in more detail if you have wired your layout to
allow it.
The whole question of turnout feedback is then "How do we configure the program to
understand the known state properly, given my layout hardware?"
If you go to the Turnout Table tool, you'll
find it has four columns associated with feedback:
- "Feedback"
- This is the "known state". You can't change it, but this column will show you what the
program thinks it is.
- "Mode"
- This is the feedback method used by this turnout. (You can change it for each turnout
individually, but note the change doesn't take effect until you click somewhere else, and
you might need to add some sensor names in the next column(s))
- "Sensor 1", "Sensor 2"
- These define the sensors needed by certain types of feedback. You can type a sensor
number, system name or user name here; the program will change it to the system name that
it needs.
Available feedback modes are:
- DIRECT
- The default in many cases, and also the original behavior of the program. When
something tells the turnout to change, it's just immediately assumed that it actually did
it.
- ONESENSOR
- The turnout watches the sensor defined by the "Sensor 1" column, which is probably
hooked to a microswitch on the turnout or similar. When the sensor is Active, the turnout
is known to be in the "Thrown" position. When it's Inactive, the turnout is known to be in
the "Closed" position.
- TWOSENSOR
- The turnout watches two sensors defined by the "Sensor 1" and "Sensor 2" columns, which
are probably hooked to microswitches at either end of the turnout's throw bar. When Sensor
1 is active, the turnout is known to be thrown. When Sensor 2 is active, the turnout is
known to be closed (normal).
- DELAYED
- When something tells the Turnout to change, it immediately starts the operation and
goes to the INCONSISTENT state to reflect that the points are moving. After a few seconds
delay, the operation completes and the final state is reported.
- MONITORING
- Default for LocoNet and XPressNet turnouts, this means the network is monitored for
messages about the status of the turnout. This mode is only available for certain
protocols, i.e. some protocols can't do the monitoring properly, and don't have this
choice.
If selected for the NCE Powerhouse Pro (only), it will monitor the last known state change
of this turnout made by the command station (on request from a cab, macro, JMRI or Mini
Panel).
- INDIRECT
-
Currently only available for LocoNet turnouts, this informs the program that the turnout
is being driven by a Digitrax DS54 with a microswitch attached to the switch lead. For
more information, see the DS54 page.
- EXACT
-
Currently only available for LocoNet and XPressNet turnouts. For LocoNet turnouts, this
informs the program that the turnout is being driven by a Digitrax DS54 with two
microswitches attached to the switch and aux leads. For more information, see the
DS54 page. For XPressNet turnouts, this
informs the program that the turnout is being driven by an LS100 and the feedback inputs
have been connected as described in the LS100 manual (external link,
PDF).
- SIGNAL
- Recommended setting for Signal Mast controled by driver Matrix output with XpressNet.
The program does not wait for turnout feedback and sends matrix commands as soon as
possible.
Most people will (and probably should!!!) stick with the default feedback type that the
software selects for them.
Operation
When feedback is used, Turnouts can start to behave in complicated ways.
Simplest Case
The most common case is JMRI commanding turnouts to move, and they move properly. The
sequence is then:
- JMRI commands the Turnout to move by setting the commanded state to e.g. THROWN.
- The Turnout object figures out the right commands to send to the layout hardware, and
works with the rest of the program to do that.
- The position changes on the layout.
If no feedback (really, DIRECT feedback) is in use, the known state of the turnout will also
change in step 1. At that point, icons on panels, signal logic, etc will all be notified of
the change and react.
Simple Layout Feedback
The simplest case for using feedback from a microswitch on the layout is similar:
- JMRI commands the Turnout to move by setting the commanded state to e.g. THROWN.
- The Turnout object figures out the right commands to send to the layout hardware, and
works with the rest of the program to do that.
- The position changes on the layout.
- A microswitch detects that position change, and informs the layout electronics, which
in turn changes an input to JMRI.
- That input is connected to a JMRI Sensor object, which changes its state e.g. from
INACTIVE to ACTIVE.
- Because the Turnout is watching that Sensor for ONESENSOR feedback, when the change
occurs it sets the known state of the turnout to THROWN.
This sequence takes a little time, but the known state of the Turnout isn't changed until the
points have actually moved on the layout. Then, icons on panels, signal logic, etc will all
be notified of the change in known state and react.
Uncommanded changes on the layout
Feedback allows JMRI to detect that something has changed due to action on the layout,
instead of a command from the program. For example, if you're using feedback from a turnout,
you might see something like this sequence:
- The program sets the commanded state to THROWN, and instructions go out to the turnout
telling it to move to that position.
- Later, the feedback information comes back to indicate that the movement has taken
place. This sets the known state in JMRI to THROWN.
- Later, and independently, you move the turnout on the layout to the CLOSED position,
perhaps with a ground throw or local pushbutton.
- This causes the feedback information sent to JMRI to change.
- Because of this, the known state is changed to CLOSED.
- At this point, JMRI makes the assumption that you wanted this to happen; in effect, a
command was given on the layout (not in the program) to move the turnout. So JMRI changes
the commanded state to CLOSED also.
Note that this last transition in commanded state does not send commands to the layout; it's
just a change within the program.
TWOSENSOR feedback and inconsistent state
TWOSENSOR feedback uses two separate sensors to indicate when the turnout has successfully
gone to CLOSED and THROWN state. During the transition between those two, it's in neither
state, so the turnout state appears as INCONSISTENT. For example:
- The turnout is currently showing CLOSED, with the closed-sensing Sensor showing active
and the thrown-sensing Sensor showing inactive.
- You command the turnout to move, and it starts moving.
- The closed-sensing Sensor going inactive as the points lift off, and the turnout starts
to show INCONSISTENT.
- The thrown-sensing Sensor goes Active as the points settle on closed, and the Turnout
will start to show Closed.
The same happens if you remotely start the turnout moving, for example with a local
push-button.
- The turnout starts to move
- The sensor indicates that by going inactive, and the turnout shows INCONSISTENT
- The other sensor sees the turnout settle into place, and the final state is shown.
- JMRI thinks you must have done that intentionally, and sets the commanded state equal
to the feedback state.
(N.b.: Sometimes the mechanical linkages are not perfect, and the "to" sensor will go Active
before the "from sensor goes Inactive. The program handles this the same way: It will show
INCONSISTENT if either both sensors are Active, or both Inactive)
Bus-based systems and input messages
Some DCC systems, e.g. LocoNet, XPressNet and NCE systems, can handle commands to turnouts
from multiple sources. When set to MONITORING feedback, JMRI looks for those for confirmation
of the proper state of the turnout: Seeing the message sent is considered confirmation that
the turnout has changed.
With MONITORING set, when something outside of JMRI (e.g. another program; a handheld
throttle) changes the turnout state, JMRI will accept that and change its internal state for
the turnout.
