Brian Podmore

The final part of this project brings together my travelling Post Office coach, the ground apparatus and the use of a digital control system to control a successful drop and pick-up of mailbags. This was a move on from the essentially mechanical operation of the old Hornby TPO set designed to run in a DC environment which I still have. So, I hope you find what follows of interest. I was no expert in the use of DCC apart from setting up locos to run on my railway. My advice therefore, before making a journey into the so called ‘dark arts’ of DCC, (Digital Command Control), is be sure of what you want to achieve. Spend time researching and asking questions about how your endeavours can be realised by using a DCC system. Get advice on the types of decoders you should use and learn how this can work with the control system you want. I can’t stress how important it is that you have the support of knowledgeable and helpful suppliers who have the experience, and the technical know-how, to help you along the way.

My starting point with DCC and loco control

I have been using DCC for some years, first of all in 4mm using Lenz equipment and computer control, and subsequently with ZTC and ESU components, so, based on this experience, I could see that the DCC approach was the way forward on this project for me. Nevertheless, how to do it was beyond me at that time. One of the advantages of DCC is its ability to control both individual and multiple events on a railway, and there are products on the market that do this admirably, from point and signal control to routing and computer control.

As I was already using ESU decoders in my locos, I opted to use a Loksound XL4 decoder inserted into the TPO coach to control its functionality. A particular facility afforded by this decoder is the way in which servos can be connected to it, and no soldering is required. This turned out to be advantageous at the trialling stage, where putting together and taking apart of the components occurred on a regular basis. This particular decoder can control up to four servos, although I would only be using two. I also purchased two ESU servo packs as shown in Fig 1.

Figure 1

They come with a sturdy fixing bracket that allows the servo itself to be setup in several different positions. The servo packs also contain several types of lever arm, making them useful for a range of situations. Bear in mind there are always cheaper alternatives and different ways of achieving the same outcomes, but this was the way for me. Fig 2 shows the servos out of the packaging and placed onto the coach chassis, this is roughly how they were positioned.

Figure 2

You can see the fixing brackets here that are included in the pack. I created a wooden floor for the coach and glued it down so that I could screw the servo brackets securely to it. Everything had to be well fixed to absorb the impact shocks when dropping and collecting mailbags.

The circuitry for the decoder, servos and speaker was constructed as shown in Fig 3a.

Figure 3a

Figure 3b

On the left, the input feed from the rails was enabled through the left and right hand wheels of the bogies which were insulated from the coach chassis. On the Loksound XL4 decoder, as previously mentioned, there are four special connectors for servos using a standard socket arrangement, (I used two of these). One to extend and retract the traductor arms, the other to open and close the net. I also wanted the decoder to control other items such as a warning bell and internal lights.

Fig 4 shows one of the servos in its correct position.

Figure 4

Complete with the connection to the arms of the traductors. If you are a DCC expert you can probably skip some of what follows, but if you are new to DCC or even only familiar with basic control using it as I was, I will try to explain the principles involved in a straight forward way and describe the eventual success it brought me, but it’s important to remember that different control systems vary in how they setup and execute their commands to decoders.

The DCC environment beyond loco control

To start with, the way to set anything up using DCC is by programming in certain values held in the memory area of the decoder. These values tell the decoder how to operate various objects connected to it whether it is a motor, a light or making sounds. The decoder’s memory is like a large number of pigeon holes in which numbers can be stored, (not unlike the letter racks on the TPO). These pigeon holes are known as CVs (Configuration Variables). There are many CVs and depending what value they hold they control all sorts of things in a particular way. Generally each make of decoder has these CVs and each make is supposed to do the same thing but there can be variations. Programming the decoder is accomplished usually by putting the loco, (or in my case the coach), on a ‘programming track’. This is connected to the part of the control system command base in use that can see and change these CVs. ESU also sell a programming board that directly changes the CVs of a decoder before it is put into use, or you can pay someone to do some or all of it for you. To change a CV you would normally dial it up in some way on the command base keypad, key in its new value and then save the changes to the decoder. This is usually done by pressing a combination of keys and entering the new values, care is needed in doing this and the decoders accompanying manual usually explains how this process is carried out.

Configuration variables in more detail

Each CV itself has a range of numbers it can be set to, (as with ‘loco address,’(which is CV1), the range is 0-127 or ‘maximum speed’ (which is CV5), the range is 0-255. In the case of the servos there are a set of three CVs that control the start position, the end position and the speed with which the servo rotates. But having said that there is always a trial and error process that you go through in determining which values are right for the decoder you are programming. I have a fairly small stock of 20 locos and they all have different settings. They have different gear ratios, different wheel configurations and ‘chuff’ rates and so a one-size-fits-all cannot apply. Having said that, the following is an account of how I set up this particular system in a little more detail.

Setting CVs to function Buttons

Although this coach is part of a consist, it is still a separate object within that train and in order to take control of the coach in the same way I would normally take control of any loco, I merely have to dial its number up on the controller. Therefore, I set the ‘loco’ address, (CV1) of the TPO coach to be 20. Also, I want to be able to extend and retract the traductors by pressing a particular function button, also extend and retract the net by pressing another function button and finally switch on or off lights by pressing a third function button all from my controller. I also want a warning bell to sound simultaneously while the net is in use. I have a video of this working on YouTube: https://youtube/7CDOW5HZxI Therefore, for this coach I set up the following function keys:

F1 button: to control the Traductors

F2 button: to control the Net and the warning bell sound

F3 button: to control the lights.

I also set the function buttons so that they have to be pressed momentarily once to start and then once again to stop an operation, not to keep them held down, (during the operating window the traductors and net are activated almost simultaneously and trying to press two button at once wouldn’t work on my system), also I think it’s a safer option when operating the TPO. I will refer to button pressing again below when working with alternative systems such as JMRI, (Java Model Railroad Interface). As I was no expert in this area, I had a great deal of help from Coastal DCC, based in Ipswich, to whom I am grateful for the excellent advice and support I have received from them during the process of putting this project together. It was a learning curve for me and this is just the kind of support our hobby needs if it is to continue to thrive.

So, what does this look like for one of the servos?

Making the necessary connections

First it’s necessary to set up the servo to work from a connection on the decoder, (I used one of the four connections labelled AUX 7,8,9 and 10), and this would be controlled by pressing the F1 key on the controller. The effect of this is to tell the Loksound 4XL decoder to do something with the servo in a precise and particular way, (fig 3b). This can be done by the process of what is known as ‘function mapping’; you can do this yourself by using a piece of ESU software, (free to download from their web site), but it will require the purchase of a programming board from ESU. Alternatively, the seller of your DCC decoder should be capable of setting up this part for you, as indeed it was done for me.

Important note: My control system is a ZTC 505 base station. This 505 won’t communicate with a DCC decoder unless it has a motor fixed to it, (which it normally would if placed inside a locomotive). So while I’m programming the decoder I connect a small motor to the appropriate connections. Once changes are complete I remove it.

I mentioned that a decoder could send information to a servo to control its start and end position and also its speed. Once the Aux socket has been set up, this can be done without any extras needed. It is a matter of trial and error though, until you are satisfied it will work correctly and safely. It has to be done in situ to get it right. Get it wrong and you could physically damage your good work, removing and replacing the decoder from the coach and measuring and adjusting the amount of rotation of the servo arm is all part of it.

Setting the CVs to control the servos

So what CVs are involved? What follows is an example of the values of the CVs necessary to make the connection on AUX 7 control the servo worked by the F1 button, (the CVs specific to this are 326, 327 and 328):

CV 326, (sets the speed of this servo where the lower the number the faster the servo rotates) I set this to 10. (Servos are geared to make them powerful and move slowly.

CV 327, (sets the start position of this servo), I set this to 0.

CV 328, (sets the end position of this servo), I set this to 36.

Setting CV 328 to 36 gives an approximate 90 degree turn, and setting it to 18, an approximate 45 degree turn. These values will be dependent on the start position you choose of the servo arm, the amount of rotation required and the plane in which it is working. You will have to remove and then fix the arm back onto the servo to get the correct start position for the arm; it does work with a little perseverance, as I have alluded to before it’s not an exact science!

Consistent control issues

A slight aside here but important in discussing how buttons can be made to work as either a momentary touch or a continuous press. One system I use to control my railway uses JMRI software on an iPod touch. Again without being too technical, an old laptop running XP has the free JMRI software installed on it and it is connected to my DCC system via a ZTC computer interface box. I use a wireless extender on my home network that allows my iPod, (another redundant item), to communicate with the network that the laptop is connected to. In this way the laptop can see the iPod Touch and using software, (WiThrottle also free from the JMRI stable), I can control my locos etc. wirelessly, but, I had to make an adjustment to the JMRI set-up for this to work for the TPO. (I don't use it for anything else except to drive my locos as an extra handset controller, but it is a whole new world in computerised railway control if you want it to be). This meant changing a value on the F2 set-up in JMRI as it is usually ‘momentary’ and it is assumed that you will only ever use it to blow your whistle or horn, and not use it continuously, which means you have to keep the button pressed to make it work.

The touchscreen of the iPod was not ideal for operating the TPO in this way and certainly not what I wanted. By changing it to continuous, I can now operate the TPO in the same way as on my other controllers. As soon as the traductors have off loaded their bags they can be retracted, followed shortly after by the net once it has collected its bags and before it enters a tunnel. If nothing else it’s a safety thing, and you don’t get tied up with cable!

Back to the physical aspects of the coach

Fig 5 shows an internal view of the final setup.

Figure 5

The servos are screwed into the floor as is the decoder. The wire connections to the net and traductors can be seen and the CVs have been adjusted to allow for the differences in operation of the traductors and net. For instance the net has further to travel and needs to do that quicker than the traductors, hence there will be different values needed for the relevant CVs, (as stated earlier your DCC supplier should be able to help here).

I don’t propose to go into any more detail here on setting up all the CV values. I’ve just tried to give a general outline of what is required. There is a fuller explanation on my website www.bpodmore.co.uk, use the search option to look for ‘TPO’ and it’s under one of three headings halfway down the main TPO page.

I've also put in some LED strip lighting at each end of the coach to illuminate the 'posties' in their working positions (Fig 6 and 8).

Figure 6

This is an LED strip of three lights with inbuilt resistors from CRT Kits. All that was necessary was to set up the CV values to make the lights come on using the function 3 button on the controller.

Finally, in Fig 7 and 8, you can see in more detail both the working areas including the lighting and a small section with pigeon holes for letters with our three postmen working hard to deliver the mail.

Figure 7

Figure 8

In practice the pigeon hole arrangement would extend throughout most of the coach but spectators will not be able to see that here. The postman kneeling and securing the bags to the traductors only has an iron bar to protect him from the elements. Accidents did occur and men were known to fall off TPO coaches with dire consequences. There were clear instructions laid down on how the operation of the coach was to be carried out; when the net and traductors were to be lowered and after disposal and collection when they were to be raised. Failure to carry out these instructions to the letter could result in a major accident and injury or death to employees or passengers and significant damage to the train. This is a far cry from the starting point for me which was the Hornby 3-rail TPO set!

And finally...

To illustrate a TPO working, here is a fictitious example schedule for the mail coach crew. Remember this happened often at night time as well and at speeds in excess of 50 mph:

1,560yds east of Warmsby station, pass through two tunnels.

Pass over a viaduct and by two platelayers’ huts.

After passing a distant signal fix on the bags whilst running through a cutting.

Pass through Dunsby station and after passing the starter signal at the end of the platform lower the net and extend the traductor arms, the warning bell sounds continuously.

Stand clear of the apparatus whilst the bags are dropped from the traductor arms and other bags are collected in the net.

Retrieve the net and traductor arms immediately after execution and before entering the third tunnel.

However the TPO is controlled, it really is a team driven activity. One person drives the locomotive, another controls the actions of the TPO. It requires concentration and timing on the part of the mail coach operator.

When I run a mail train on my railway there is always a slight tension before executing the operation with the hope of a successful outcome. Anyone taking on the responsibility of TPO controller has to have successfully completed a training session. It adds to the realism and is an out of the ordinary episode that makes this hobby both a challenge and a rewarding pastime. So what’s the next challenge? A slip coach?

Now there’s a thought…

Suppliers:

Coastal DCC, 71 Alan Road, Ipswich IP3 8EY Tel: 01473 403764 Web: www.coastaldcc.co.uk

South West Digital Ltd, 202A High St, Worle, Weston-Super-Mare, North Somerset, England, BS22 6JD Tel: 01934 515382.Web: www.southwestdigital.co.uk

CRT Kits, (LED strip lighting) Poplars Farm, Aythorpe Roding, Dunmow, Essex CM6 1RY Tel: 01279 876402 Web: www.crtkits.co.uk