Hornby Select

[Chrissaf]

Welcome Richard

The actual voltage you see depends upon if there is a load on the track and the spec of the meter you use but 15vAC is about right for most meters.

 

Download the latest manual covering v1.5 from the main site download manuals area and try a reset on your Select - part of the procedure will offer the option to disable loco zero (analogue) running (see -0 on screen).

 

What you described is DC runaway and as noted if you put a DC loco on track it will protest by buzzing. This is not good for the motor and you are advised never to run a DC loco on a DCC track. Note that you should also disable DC running in the decoder via CV29 but the Select cannot do this until v1.6 which is due out soon.

[Chrissaf]

Hello everyone, I am new to DCC and bought a second hand Select to get started. I had it reflashed to version 1.5 at Hornby

Now there are problems. Is the DC function disabled. If I put a DC loco on the test track, it shoots off to the right at full wallop. Also the voltage across the track is showing 26v AC  I had an idea that it was about 15v. So I have onplugged it.

Just plugged in and tried it again and its 15.8v across the track and the DC loco runs on the knob, so to speak. with a buzz. I dont intend to use the DC function but at least its back to how it should, or is it? 

Any help would be gratefully received, its driving me bonkers, nothing seems to be going right. Toot Toot Richard

[richierex]

Thanks for the information, It is strange that using the same meter, it showed 26v when the problem occured but then 15.8 ( which I know is about right ) about 1/2 hour later. I will now check all connections as I think the Hornby track connector with the push in fittings is a bit rubbish. Also all soldered joints as it seems as if the DCC loco is losing the digital signal intermittently and carries on at whatever speed was set. I have already downloaded the 1.5 manual and as I have never used earlier methods, I should be ok

[2e0dtoeric]

Richie - I'm not quite clear on what you are saying. Dgital loco's are supposed to carry on doing whatever they were instructed to, until you tell them differently! That's how you can operate several simultaneously!

They cannot 'lose signal' without losing power. If the power is interrupted, everything will stop dead!

[96RAF]

There is a little foible with loco zero that can be seen when using the Select and which can affect both DC and DCC locos on track.

 

If you have selected loco zero (even if by accident) and applied throttle, then gone off to select another (DCC) loco and operated that, the stretched zero bit that operates an analogue loco will also affect the DCC signal and can disrupt operation of DCC locos. Any analogue loco on track with also respond to the previously throttle setting and assume that speed.

 

The trick is to select loco zero and wind the throttle up then back to zero before operating any other loco at the start of a session just to make sure all the ducks are in a row.

[richierex]

Richie - I'm not quite clear on what you are saying. Dgital loco's are supposed to carry on doing whatever they were instructed to, until you tell them differently! That's how you can operate several simultaneously!

They cannot 'lose signal' without losing power. If the power is interrupted, everything will stop dead!

Hi, Its as if it is not recognising the digital signal, only the preset speed/voltage. Are you saying that these are the same, I thought the operating voltage was a pulsed signal and the digital instruction was over this, seperate. Back to school for me then. I have a background in telephony, and VOIP protocols so have a grasp of it Many thanks

 

[Chrissaf]

Hi, Its as if it is not recognising the digital signal, only the preset speed/voltage.

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Richie,

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DCC is Digital. The controller sends a series of binary 1s and 0's in a code that is recognised by the DCC decoder and converted into a command. The [preset speed/voltage] voltage of the DCC signal plays absolutely no part in directly controlling the loco speed and direction. I think I know what Eric above was trying to say, but perhaps poorly worded it. You can lose DCC signal integrity without loosing power. The tutorial below may help explain this.

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The voltage you can measure is a bi-polar square-wave voltage of 28 volts peak to peak. As it is not a sine-wave, any normal voltmeter on an AC scale will try to interpret the voltage as best it can. Most meters read a value of about 14.7 volts AC.

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The controller sends a pulse, the width of which determines whether it is a Digital binary 1 or a binary 0. The image below shows the DCC signal on a scope.

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Bi-polar means that the signal has voltage either side of the zero voltage line. Thus both the positive and the negative half cycle contains the same binary information. This is why the locomotive always travels forward (when given a forward command) regardless of which direction the loco faces on the track. Because the ability of the decoder to read the Bi-polar data is not polarity sensitive.

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The frequency of the DCC signal is a variable, because the timebase of the waveform changes depending upon the actual binary 1 and binary 0 pattern being transmitted. The nominal frequency is 7Khz (7,000 cycles per second). The power to operate the motor is derived from passing this 7Khz Bi-polar signal through a bridge rectifier, which converts it into a form of DC that the decoder can use to control the motor and other DCC functions such as lights and sound. The binary 1 and binary 0 pattern when decoded by the decoder tells the decoder how to apply the rectified voltage (magnitude and polarity) to the motor, using DC PWM (Pulse Width Modulation) which is a completely different waveform method to the raw track DCC Bi-polar signal. For one thing, PWM is not Bi-polar. PWM pulses are either all positive or all negative on one side of the zero volt line and contain no digital information. PWM is purely analogue in nature.

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If for any reason the signal becomes corrupted. Then situations can arise where the decoder cannot understand the DCC command being given to it. So it continues to perform the last clear uncorrupted DCC signal that it did understand. Say for example, travel forward at speed step 35 out of 128 speed steps in total. Therefore as long as the Bi-polar waveform is present, it can still be bridge rectified to provide locomotive motive power even if it is so distorted that the decoder cannot understand it as a digital command.

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So what can cause this corruption, such that the decoder loses the ability to synchronise to the data being sent to it?

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Answer....lots of things!

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• Dirt on the track and wheels is the most common cause. The binary 1's and binary 0's are very short pulses, measured in micro-seconds. Thus a break in power transmission of a few milliseconds caused through dirt and grime can cause a lot of DCC packet data to be lost, but at the same time a power break of a few milliseconds is unlikely to stop a loco. This is one reason why DCC locos are more sensitive to track and wheel dirt than DC Analogue locos.
• Another common problem that comes up a lot on this forum is incorrectly using DC Analogue power track / clips on the DCC layout. These power track/clips contain suppression capacitors that are connected across the track. Capacitors pass AC frequency, so although they have no effect on DC Analogue control of locos, they can corrupt and distort the Bi-polar DCC signal to the extent that the command cannot be understood by the decoder. Note that the Hornby Select controller is particularly sensitive to these incorrectly fitted power suppression capacitors in the DC Analogue track products. This includes the Hornby R8201 Link Wire kits.
• Induced noise onto the track from nearby electrical devices that are poorly suppressed can also affect the clear clarity of the DCC signal on the track.

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But none of these problems affect the ability of the decoder bridge rectifier from converting the resultant corrupted DCC signal into motive power for the loco. So as previously stated, the loco continues to perform its last received command that it correctly received and understood.

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In some circumstances, the DCC signal can be so badly distorted and corrupted that the decoder loses the ability to even detect that a DCC signal is present. The decoder can then incorrectly switch itself into 'DC Operation' mode. It then interprets the full voltage DCC signal as being a condition that is equivalent to being a full speed DC track voltage (remember that the Bi-polar DCC signal is bridge rectified into a form of DC for consumption by the decoder). This makes the loco shoot off at full speed with a resultant loss of digital command control.

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Most devices that use digital communications have a very similar digital packet structure. The packet is typically made up of four elements:

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1. Preamble (used to train the receiving device to identify the start of the actual data).
2. Address (to identify what device needs to respond to the data being sent).
3. Instruction (to tell the receiving device what it needs to do).
4. Error Detection (to confirm that the receiving device has correctly received all the data).

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The image below is pasted from the NMRA DCC Standard for the basic DCC General Packet Format.

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/media/tinymce_upload/90115f08570a1d91bf22fb313e84a6aa.jpg

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The image above is a very simplistic representation of a DCC Packet. The NMRA DCC standard has been amended over the years to add the capability of new features. This has resulted in today's DCC devices usually adopting the more complicated DCC "Extended Packet Format" which has variable length up to 6 Bytes plus Preamble.

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As you can see from the above, it is too simplistic to treat the DCC track voltage and the data it contains as being one and the same thing.

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Hope this tutorial aids your understanding, and for anybody else who has an interest.

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PS - Can I ask you NOT to use the 'White Arrow in Blue Button' as you did in your last reply. The 'Blue Button' is NOT a 'Reply to this post' button and using it makes it very difficult to see what is the new text you have added.

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Scroll down to the bottom of the page and write your reply in the empty 'Reply text box' and click the Green 'Reply' button.

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[96RAF]

Just to add to Chris’s explanation. The extended zero bit that an analogue loco uses changes the ‘mark/space’ ratio of the binary 0 such that the positive or negative part is more prominent than the other and this causes a DC loco to see it as either a positive going or negative going potential and reacts with speed and direction accordingly.

 

Going back to what I said about loco zero being left with throttle applied leaves this binary 0 in a biased potential which can affect a DCC loco.

[Fishmanoz]

Chris, could you please reconsider your text equating analog, DC and a PWM signal.  Although I do get your reason for saying so.

 

A quick Fourier analysis will put the lie to a PWM signal being DC.  It is a square wave of relatively high frequency with modulated or varying mark space ratio.  In fact, in this regard it is analogous to the stretched zero bit that Rob describes for address 0 DC loco operation.  In both cases the motor sees this variable mark space ratio square wave but, being highly reactive, filters out all of the high or non-zero frequency components (again Mr Fourier can help here) by being (almost) open circuit to them, leaving only the average DC value to drive the motor.

 

However, just as the DCC frequencies are high but not infinite, so the motor is not actually open circuit to them but high impedance, so is affected by them to some extent.  Hence the buzzing you hear with a DC motor on a DCC track as the motor moves backward and forward slightly to follow the square wave frequency.

 

PS.  Fourier analysis, or frequency domain analysis, doesn’t look at how the signal varies over time, rather it looks at a particular time only and sees what frequencies are contained in the signal.  In a square wave, the frequencies present are the fundamental frequency of the wave plus various harmonics of the fundamental frequency with decreasing amplitude as the harmonic number increases.

[Chrissaf]

Since the PWM pulses are all pulses of varying width but crucially of the same polarity. Then yes it is true that they are not pure DC, and it is true that the motor derives the equivalence of an average DC value from it, and it is true that a square waveform shape contains the odd harmonics of sinusoidal frequencies, but the harmonic content are waveforms that are all voltage variations of the same basic polarity i.e either all above or all below the zero volt line and do not straddle across it. Therefore, although the PWM waveform contains components of frequency it is not an AC waveform in the traditional sense and is nearer to 'pulsed DC' when considered very simplistically.

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It was in the context that PWM was being used to control a DC motor, that the reference to "DC PWM" was made, and just trying to highlight that PWM was nothing like a native DCC Bi-polar (AC) track voltage waveform.

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Not that the semantics of PWM theory seems to matter now, as the OP has made no indication that he has even read the tutorial reply.

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