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Support: Digitrax SE74

An image of the SE74 with one signal cable attached
           and one signal connected to that signal cable

Using the SE74 with JMRI

The Digitrax SE74 provides a variety of features which may be used with JMRI. JMRI-supported features include signal control, switch control, and inputs.

Configuring the SE74 with JMRI

Since JMRI 5.3.4 DecoderPro can configure the Op Switches of the SE74 so long as it has a LocoNet connection to the SE74.

To do this, you open DecoderPro (as an individual application or via selecting "Roster" in PanelPro) and create a new roster entry. Select the Digitrax SE74 decoder type for it and enter the SE74's address. You can then use the usual DecoderPro panes to read and write the configuration variables (Op Switches) in the SE74.

Configuring the SE74's routes be done using the methods described in the Digitrax documentation for the SE74.

Signaling using the SE74 with JMRI

The SE74 has two mutually-exclusive operating modes which determine how JMRI can control the signal heads. As such, these instructions present two different ways of configuring JMRI to control the SE74 - one for each SE74 operating mode. One method is "SE8C-compatible" mode. The other is "Aspect" mode.

The "Turnout" mode of controlling signals makes use of Turnout control messages to convey the "aspect" information to the hardware. It operates in the same way as the SE8C device does, and JMRI support for this mode of operation is the same as when using the SE8C device.

Alternately, control of SE74 signal heads is via an "Aspect". This mechanism uses different LocoNet messaging than SE8C mode, and requires different support in JMRI. SE74 "aspect" mode signaling has a few benefits over SE8C mode. SE74 users are encouraged to make use of this mode of SE74 operation.

Signaling when the SE74 is in "aspect" mode

The SE74 offers an alternate mode for controlling signal head appearance. In the SE74 documentation, Digitrax refers to this mode as "aspect" mode. Control of each individual signal head is via DCC "Extended Accessory Decoder" packets. LocoNet-connected SE74 devices may get their control information directly from the LocoNet messaging, rather than from the DCC track signal.

The DCC "Extended Accessory Decoder" messaging uses an "address space" which is independent of the addressing used for turnout control messaging. As such, a turnout with address 260 does not conflict with a SE74 signal head at "aspect"-mode signaling address 260! This also makes it easier to configure SE74 addressing in a way that does not conflict with other "turnout"-type objects associated with the layout.

When an SE74 is operating in "Aspect" mode and it sees such a message which is addressed to one of its signal heads, it extracts the aspect number from the message and updates the addressed head to reflect the aspect associated with the aspect number. Note specifically that the "Aspect" mechanism Digitrax uses is controlled at the level of the "Signal Head". This design choice provides compatibility with some 3rd-party LocoNet signaling hardware. And it means that JMRI does not require SE74-specific support in its signaling implementation. It also means that changing the aspect of a mast which has more than one head can require more than one message to be issued when the aspect changes. (When a JMRI Signal Mast's aspect changes, the JMRI implementation will send a message to each Signal Head associated with the Signal Mast, regardless of whether the head aspect has changed).

This SE74 behavior is notably different from the behavior seen in some other signaling implementations. Some devices use a single single DCC extended accessory decoder control message to influence _all_ of the heads on the mast. That difference in hardware implementation and messaging usage means that configuring a JMRI Signal Mast using other hardware types will be different than configuring a JMRI Signal Mast for the SE74-controlled signal heads when the SE74 is configured for "Aspect"-mode signaling.

Signaling when the SE74 is in "SE8C-compatible" signaling mode

One of the SE74 signal control modes relies upon DCC Accessory Decoder (in other words. "Turnout") control mechanisms. The SE74 Instruction Sheet refers to this mode of operation in several different ways: "4 Turnout" mode, "SE8C Compatible" mode and "SW control" mode. For the purposes of this documentation, the use "SE8C" mode shall be used.

As is implied by one of those names used in the Instruction Sheet, this SE74 mode uses the same LocoNet messaging and mechanisms to control the signal head behavior.

When operating in "SE8C" signaling mode, the SE74 may be used JMRI when using any DCC system which supports control of turnouts from JMRI.

Which SE74 mode should I choose?

There are a few considerations when choosing an SE74 signal head control mode:

In the author's opinion, "Aspect" mode is preferred over "SE8C Compatible mode" for any system which can support "Aspect" mode.

Configuring SE74 "Aspect"-mode "Signal Head" objects in JMRI

This section describes how to associate a JMRI "Signal Head" object with the SE74 hardware and one of its controlled Signal Heads, when the SE74 is configured to use "Aspect"-mode signaling. This section does not apply when the SE74 is configured for "SE8C"-mode signaling; see this section for information on configuring JMRI Signal Head objects when using the SE74 in "SE8C"-mode.

It is assumed that you have JMRI configured to communicate with your DCC system, and that you have configured the SE74 for "Aspect" mode signaling and that the SE74 has a base address which does not conflict with any other aspect-mode signaling hardware. With those assumptions met, you may start configuring the JMRI "Signal Head" objects.

Within the JMRI Panel Pro application, open the Signal Heads" table (i.e. Tools ⇒ Tables ⇒ Signal Heads ). This table shows all of the Signal Heads which have been defined.

To configure a new JMRI "Signal Head" object, it is necessary to "add" a signal head. This is done by activating the "Add" button at the bottom of the Signal Heads table. This will open an "Add Signal Head" window, where the specific information required to define a signal head may be entered. An example image is shown below.
Example of the add signal head window when adding a DCC controlled Signal Head

Note that there is no JMRI requirement to select a user name that mentions the Digitrax "head letter", except that it will be easiest to wire and debug if you leave yourself sufficient hints. Those hints may be left in the user name, in the "comments", or may be left out of the JMRI information altogether at the user's choice. If you are able to compute the connector number and Digitrax "head" letter from the "head" address and the SE74 base address, you may not need such hints.

Note also that the SE74's "aspect-mode" signaling provides a SE74 setting where the "Lunar" aspect may be made "persistant". JMRI is not able to understand the concept of a "persistant" aspect capability, so that feature needs to be disabled in the SE74's OpSw settings.

Configuring SE74 "SE8C"-mode "Signal Head" objects in JMRI

It is assumed that you have JMRI configured to communicate with your DCC system, and that you have configured the SE74 for "SE8C Compatible" mode and that the SE74 has a base address which does not conflict with any of your other hardware. With those assumptions met, you may start configuring the JMRI "Signal Head"s.

Within the JMRI Panel Pro application, open the Signal Heads" table (i.e. Tools ⇒ Tables ⇒ Signal Heads ). This table shows all of the "JMRI Signal Heads" which are defined within JMRI. A JMRI "Signal Head" provides a mechanism for JMRI to interact directly with a single signal head.

Set up each individual signal head by "add"ing a signal head. This is done using the "Add" button at the bottom of the Signal Heads table. This will open an "Add Signal Head" window, where the specific information required to define a signal head may be entered.
Example of configuring a JMRI Signal Head for a SE74 configured for SE8C-compatible signaling mode