QSI Keepalive

This page under construction

Overview:

The question of keepalive use has come up several times. I will focus on the LS Titan, although I will mention the HO Titan U

Basically a keepalive is a storage capacitor that stores charge and supplies it to the decoder when power to the decoder is interrupted.

The idea is to keep the decoder "alive" during power interruptions.

But how it does this, for how long, and what functions "continue" during the power interruption vary, and is the cause of a lot of confusion.

How the keepalive is connected is also a source of confusion and a lot of misinformation

Description of Keepalive types and construction

So the basis of the Keepalive is an electrolytic capacitor. The term "super capacitor" usually is applied to a compact, high capacity electrolytic, and NORMALLY they are low voltage. For now, ignore all that nomenclature. Also often several capacitors are connected together to achieve higher voltage and/or capacitance.

Capacitance is expressed (normally) in mfd which stands for Micro Farads.... also shown with the specific "micro" character: "µ" as in µF. Typical small scale keepalives are in the hundreds for thousands of mfd, or close to a farad in larger scales. Again, often several physical capacitors are connected together to increase capacitance and/or working voltage.

Two terminal keepalive:

Often 1 or 2 other components are added... a low value resistor is often in series with the cap bank. This limits the "inrush current" when the unit first charges up from a discharged state. This momentary high current is often interpreted as a short circuit and will trip the breaker. The resistor "limits" the maximum current, so it reduces this effect. 

Also often there is a diode "across" the resistor... when supplying current, the resistor will "eat" a bit of voltage, and the diode is basically a one way valve to provide less resistance when discharging..

Three terminal keepalive:

This was invented to get around a major issue with a keepalive: the action of the keepalive can interfere with service mode programming, due to the low voltage and currents used in this mode. 

Therefore a 3rd wire was added as a "control input" to effectively connect or disconnect the charge/discharge capability, and then the decoder can effectively disconnect the keepalive when it could interfere with operation, namely service mode programming.

More variations:

There are keepalives with more sophisticated circuitry on board, for example a DC to DC converter to allow charging the cap bank to a higher voltage and/or more quickly. This adds cost and size, but it still presents itself as a power source when needed.

How the Keepalive is applied:

Even more variations! There are multiple ways to connect the keepalive power to the decoder.

The simplest is right "after" the full wave bridge that all decoders use to rectify the square wave AC on the rails to DC. This has the benefit of powering the decoder in entirety, properly configured, everything stays "on", i.e. the "brains" of the decoder, i.e. the state, the lights and the motors. Clearly this requires the greatest amount of stored energy and also the highest voltage. 

The other common way is to power the microprocessor. Almost all DCC decoders use an internally created and regulated 5v supply to run the micro. Connecting a keepalive here will keep the micro from resetting, but no motor power. Also, depending on the design, lights will not be powered and the sound might not. This has to be checked on a case by case basis. The benefit is you can have a longer power interruption with less power. This helps a lot in the smaller scales. Also it might be best in some large scale cases when the motors are drawing 3 or so amps, which will normally discharge the keepalive very quickly.

OK, so finally we can get on the the QSI!

One of the LS Titan pinout diagrams shows a LENZ keepalive connected, and it is the 3 terminal type, see below:

You will see that at the bottom, there is a Lenz Power 3, with the plus and minus to J19 and J20... these connections are to rectified track voltage, just like the connection diagram I saw on the HO decoder.

So the first thing to do is enable the keepalive with bit 6 on CV56.0.

This worked for Colin S. I'll test it on my LS, but no reason it won't work.

The next thing I will test is how CV4 works, per the manual, it should use the keepalive to keep the motors running for a while (triggered by loss of DCC signal), determined by the CV4 momentum setting.I'll report back here on my findings.

Next I'll see if changing CV58.10.0 and CV58.10.1 will extend that time as per the manual. Again will report back here

3 wire keepalive on Titan

According to QSI wiring diagrams, the keepalive control pin is on J2-11, which is port 10 on the LS Titan  (on the titan U HO decoder, the documentation says defaults to light port 10 also)

so now, the next part of the investigation is reading out what FEATURE ID the default firmware has on port 10

This should be CV 115.PI.SI, and reading all 115 CV's, look for the one that is pointing to port 10, in the LS case, I found this was step lights.

Does not seem to make sense yet... 

So, time to experiment on port 10..