GENERALINTEREST

Märklin Digital ModelTrain Control (1)

From an idea by J. Schröder

This circuit provides an excellent opportunity to upgrade your Märklinmodel train system from ‘traditional AC’ to digital control.

A low-budget approach

EEDTs, plus a short description of the latestdevelopments in the field.

The crux of digital model train control isthat several trains on the track can be con-trolled independently. As opposed to tradi-tionally operated tracks, where turning thespeed governor on the transformer puts allrolling stock in motion at the same time, thedigital track is marked by each locomotivehaving its own control element, allowing itsspeed and direction to be controlled individ-ually. In addition to this function, there isoften a plethora of options of the ‘bells andwhistles variety, including control of turnoutsand signals via the track. Unfortunately, how-ever, these extras and their control are out-side the scope of this article.

How does it all work, you may wonder.With traditional control systems, a voltage is

Beginning model train enthusiastsmay have more digital-ready(Märklin) locomotives moving abouton the track than they are aware of.Usually, these locomotives are sim-ply powered by the speed regulatoron the main transformer. These days,model locomotives with an internaldigital decoder are hardly moreexpensive than traditional types.That is not surprising because elec-tronic circuits are easier and cheaperto produce in large volumes than anyof the traditional reversing relays.Märklin always continued to producedecoders capable of working in ‘AC’mode as well, allowing them to beused without problems with thefamous Märklin transformer. Upgrad-ing to all-digital control is then pos-sible at a later stage. Possible, yes,but admittedly at a price becausethe cost of the upgrade will easilyexceed that of all rolling stock.

Several attempts have been madeto lower the threshold. From 1987roughly to 1991, Elektor Electronicspublished items to create the all-home-made EEDTs (Elektor Electron-ics Digital Train System), a hugelysuccessful series! Some time ago,Märklin introduced their Delta sys-tem, which is actually a strippeddown version of the original DigitalH0 with limited addressing options(4 instead of the usual 80). In fact, theDelta system triggered the author todesign the circuit and softwaredescribed in this article. ManyMärklin locomotives come with aDelta decoder fitted as standard,instead of the traditional reversingrelay. These locomotives, too, arecontrolled in old-fashioned AC mode(i.e., by transformer speed regulator),by the vast majority of model train

fans. The Digital Control discussed inthis article allows anyone and an oldPC in the attic, and capable of han-dling a soldering iron, to get the feel ofdigital model train control at a verysmall outlay. So, if you do not yethave a locomotive with a Delta (orregular) decoder, you have a perfectexcuse to step inside a model build-ing shop when it’s not December…

Recapping

Newcomers to the hobby can, ofcourse, not be expected to know allthe ins and outs of digital control formodel trains. Hence, a brief recap isgiven of what we already describedin the long series of articles on the

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Main Features- Direct connection to PC parallel port.

- Simple to operate software (Windows 3.1x, 95, 96, NT) for individual control of up to 15

model trains.

- Controls Märklin Digital H0 stock using classic Motorola data format and Delta decoders.

- Integrated compact booster, max. 3.5 A output current, with overload protection.

- Powered by original Märklin transformer or single 15-VAC

- Manual on/off control of extra function.

Figure 1. Two trinary signals on the ‘scope: The top trace shows loco address 56 (X00X),function bit = 1, speed = 7 (0001).Below, in stretched-display mode, logic 0 (00), logicOpen (10) and the start of a logic 1 (1…)

simply applied to the rails (alternating volt-age in Märklin systems, direct voltage inmost others), where overvoltage (Märklin) orpolarity (other brands) provides informationabout the direction the locomotive has totravel in. With digitally controlled tracks, therails carry a signal that alternates between afixed positive and a fixed negative level.Depending on the model train brand andgauge, this voltage usually lies between±12 V and ±18 V. The rate at which the volt-age swings from + to – represents controlinformation for individual locomotives and, ifapplicable, other devices like signals.

As with too many other products, theindustry did not succeed in agreeing on acommon standard in this field. Of the four dig-ital systems originally available (Märklin Dig-

ital H0, Lenz, Fleischmann andSelectrics), only the first two actuallygot a foothold. In addition, MärklinDigital H0 is now flanked by several‘dialects’ including the EEDTs dataformat and the ‘New MotorolaDataformat’. Other brands, too, havevariants. In this article, we will limitourselves to Märklin Digital H0,because that is the format recog-nized by the system described here.

Märklin Digital H0 employs aswitching sequence once designedby Motorola for use in remote con-trols. Information is bundled intopacks of 18 pulses. In fact, thesepulses are pairs of two pulses each.Three of the four combination possi-

bilities of the pulse pairs are actuallyemployed. 00 equals logic zero, 11equals logic 1, and 10 is logic open.In the original Motorola data format,the combination 01 is not used — inthe New Motorola Dataformat, youguessed it, it is.

A single data burst or packet con-sists of nine pulse pairs. The firstfour are used as (locomotive)addresses, supplying 34 = 81addresses of which only 80 are usedby Märklin. The remaining five pairsare only decoded in binary fashion:00 or 11; with bit 5 flagging theon/off state of the extra function, andbits 6-9 containing speed and enginereversing commands.

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BOOT1

SENSE

L6203

BOOT2

IC1

OUT1

VREF

OUT2

IN1

ENA

IN2

GND

11

10

VS

9

3

4

7

1

2

6

5

8

ULN2803A

IC3

VEE

+VS

11

12

13

14

15

16

17

18I1

I2

I3

I4

I5

I6

I7

I8

O1

O2

O3

O4

O5

O6

O7

O8

10

1

2

3

6

7

8

4

5

9

R8 4x 10k1

2 3 4 5

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

CON1 8x 10k1

2 3 4 5 6 7 8 9

R11

R9

22k

R10

47k

C8

1n

12

1311

IC2d

≥1

1

23

IC2a

≥1

6

54

IC2b

≥1

9

810

IC2c

≥1

C11

100n

C10

100n

C4

220n

J1

C5

15n

C6

15n

R3

1Ω

5

5W

R4

1Ω

5

5W

R5

10

k

R6

10

k

K3

K4

C9

10n

R1

56

0Ω

R2

56

0Ω

T1

BC547B

T2

BC547B

D2

GO

D1

STOP

S1

STOP

S2

GO

B1

KBPC601

C3

4700µ35V

C2

2200µ40V

C1

2200µ40V

T3BC547B

C7

10µ16V

R7

2k

2

D3

5V6

400mW

JP1

MC145026

A1/D1

A2/D2

A3/D3

A4/D4

A5/D5

A7/D7

A8/D8

A9/D9

A6/D6

IC4

RTC

CTC

D015

16

10

13

12

RS11

TE14

1

8

2

3

4

5

7

9

6

IC2

14

7

K1

K2

PWR

PWR

FUNCTION

on

off

U U

U

U PWR

U PWR

000066 - 11

IC2 = 4001

* *

12 - 16V

see text*siehe Text*voir texte*zie tekst*

Figure 2. Circuit diagram of the Märklin Model Train Control System. The control elements, a mouse and your PC keyboard, are notshown here.

technology, we need not concern ourselvestoo much with switching speeds or powerdissipation. At the maximum output currentof 3.5 A chosen for this circuit, the L6203remains reasonably cool. With insufficientcooling, an internal overheating protectionarranges for the IC to be switched off once acertain temperature limit is exceeded. Com-ponents C5 and C6 are so-called ‘bootstrapcapacitors’ which serve to ensure a suffi-ciently high gate voltage on the two powerMOSFETs in the upper section of the bridge.

The output current flows to ground via thesense connection and R3-R4. These resistorsserve to monitor the maximum output currentbecause the L6203 is not wholly and trulyshort-circuit proof. The voltage developedacross R3 and R4 is fed to the input of NORgate IC2b via R5-C9, a low-pass filter to sup-press inevitable switching pulses. Togetherwith IC2c, the NOR gate forms a bistablewith a special ‘treat’ in that IC2b is (mis-)used as an analogue comparator. StandardCMOS circuits are designed to switch atabout half the supply voltage. If the voltageat pin 6 of IC2b reaches 2.5 V (which happensat 2.5 V / 0.75 Ω or 3.5 A), IC2b and IC2c tog-gle.

The enable input of the L6203 is thenpulled low, the bridge is switched off, and thetracks are disconnected from the supply. Thegreen GO LED, D2, also goes out and its redcounterpart, D1, marked STOP, lights. Whenthis happens, switch S2 may be used torerestart the circuit. A stop condition may beforced by operating switch S1.

The two remaining gates in IC2 are usedto supply the bridge with the normal andinverted digital signal.

The 5-V logic supply voltage is derivedfrom the unregulated power supply. A zenerdiode is used in combination with emitter fol-lower T3 acting as a power buffer, becausethe input voltage conditions are rather uncer-tain. If, for example, a Märklin transformer isconnected and the speed control knob isturned back to the train reversing position, analternating input voltage of 24-30 V appearsat the supply inputs. When rectified, thatwould produce an input voltage surge that issure to endanger the life of a 7805 voltageregulator. With D3 and T3 included, the cir-cuit will withstand this abnormal condition.The output bridge can also safely handle thisvoltage surge of up to 52 volts.

(000066-1)

Next month’s second and final instalment willcover the system software and hardware con-struction.

Some time ago, Märklin intro-duced the so-called New Motorola’data format, in which all four combi-nation options are allowed (00, 01, 10and 11). The extra combinations inthe function bit and the remainingfour bits are used for non-volatiledirection information and extraswitching functions. The standardMotorola encoders and decoders,however, are unable to process thesepulse pair combinations.

Finally, we should mention that apause with a certain length isinserted between the 18-pulse pack-ets. This is done to synchronize thetransmitter and the receiver. Thepacket has length of about 3.8 ms,while the pause takes about 2 ms.As an extra safety measure,Motorola have built in a protocol thatarranges for the receiver to be sup-plied with the same data packettwice in sequence for it to be recog-nized as valid. This protocol appearsto be surprisingly effective for allrolling stock moving at considerablespeed across the track.

The circuit

The circuit diagram shown in Fig-ure 2 is of an attractive simplicity.The core is formed by IC4, aMotorola encoder chip typeMC14026 which looks after all con-verting into serial digital format ofdata received from the PC parallelport. In a way, the encoder IC alsorestricts the operation of the circuit: itis capable of generating the old (tra-ditional) data format only. Thisallows standard and Delta locomo-tives to be controlled. Decoders fromother brands (for example, Lenz) ordecoders having the four extra func-tion outputs utilizing the NewMotorola data format can not beused in conjunction with this circuit.

Darlington array IC3 acts as aninterface between the parallel porton the PC and the encoder chip. Thefirst four outputs are used to set thelocomotive address on the decoder.Because the open-collector outputsare not fitted with pull-up resistors,the status of the address lines isalways Low or High-Z. The ability toset a High-Z status is essentialbecause the Delta addresses definedby Märklin all have ‘logic open’ bits.If the encoder were connected

directly to the parallel port, it wouldnot have been possible to activateany Delta decoder at all!

The second nibble of the parallelport is used for setting bits 6-9 onthe encoder. These bits containspeed information and the reversingcommand: the bit combination is1000.

Bit 5, the function bit, is given afixed state with the aid of a jumperor switch (JP1) and is therefore on oroff for all locomotives to beaddressed. That should not be aproblem because this bit usuallycontrols the lighting function, whichis preferably on by default. Only theoldest EEDTs decoder employs bit 5for (non-volatile) direction informa-tion. Hence that decoder can not beused with this circuit, becausechanging the state of JP1 wouldcause all trains to reverse. LaterEEDTs loco decoder variants, includ-ing the most recent EEDTs Pro, arefully compatible with the presentsystem.

Components R9, R10 and C8determine the timing of the encoder.Resistor R9 determines the length(duration) of one packet of 18 pulses(3.8 to 4 ms), while R10 takes care ofthe synchronisation pauses betweenpulse packets.

Originally, the circuit wasdesigned for direct connection to theEEDTs Booster. However, theBooster, with its 10-amp output cur-rent capacity and considerable costand effort of building may be toomuch of a good thing, and beyondthe reach of beginners. That is whythe present circuit comes with itsown mini booster, IC1, which isshort-circuit as well as overloadresistant. To keep the cost of build-ing the project down to the absoluteminimum, it is possible to connectthe existing Märklin transformer anduse it as a power supply. Using B1and C1/C2 (or C3, see constructiondetails), a single direct voltage isderived from the transformer’s sec-ondary voltage. A full bridge outputstage has to be chosen to enable asingle-rail input voltage to be turnedinto an output voltage that switchesbetween a positive and a negativevalue. The L6203 from ST Microelec-tronics combines the required func-tions in a single IC. Plus, becausethe IC is manufactured in DMOS

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The SoftwareIn addition to this circuit, a PC is necessaryonto which the requisite Windows programneeds to be installed. Continuing in the spiritof simplicity, manual controls are not pro-vided. The system requirements have beenkept to a minimum. Practically any PC should

be capable of performing the task.This applies also to the minimumrequired version of Windows. Thetime-honoured Windows 3.1x meetsthe needs admirably. In addition,provision has been made to makekeyboard operation possible. Experi-

ence shows that, once familiar withthe keystrokes, this method of oper-ation is extraordinarily convenientand fast.

After starting the software, ascreen will appear as depicted inFigure 3. For language purists, there

GENERALINTEREST

62 Elektor Electronics 10/2000

Märklin Digital ModelTrain Control (2)Final part: software, construction and operation

From an idea by J. Schröder

Figure 3. Screenshot of the Windows program written for the train control system.

Construction

With the aid of the component overlay andthe layout of the single sided PCB (Figure 4),you will not need to resort to witchcraft inorder to build a functional circuit. A few con-struction hints, though.

There is a single wire link just above IC4.The trimmed-off part of a lead from C1, C2 orC3 may be used for this. Use this in prefer-ence to a lead from one of the resistors, since

is the built in facility to customisethe labels of the buttons to yourheart’s content.

At the top are the (fixed) locoaddress configurations: first theMärklin loco address, below that, theDelta loco address of the four applic-able controls.

The slider (can also be operatedwith the arrow keys) speaks foritself, it is used to adjust the speed.The small window above the sliderindicates the selected speed.

The purpose of the button belowthat is to select the direction of travel(toggle function). A marginal note isin order here. At certain loco speeds,this button will cause instant rever-sal of speed. At higher speeds(approximately speed level 7 and up)this abrupt reversal is disabled, per-haps for the wellbeing of potentialmodel railway passengers. In thiscase, the speed must first bereduced to a lower value, or zero,

before carrying out the reversal ofdirection.

The tick box at the bottom is usedto enable or disable the control. Theresponse time of the system is fasterwhen fewer controls are active.

The button ‘Save Settings’ savesthe configuration of the active con-trols in the file mrkln01.ini, which isin the same directory as the one fromwhich the program was started.Note that the current position of theloco control is not stored. The stopbutton (also operable by hitting thespace bar) immediately forces allcontrols to zero. In contrast with thestop button on the PCB, this stopfunction will maintain power to therails.

A number of functions, includingwhich parallel port to use (LPT1: orLPT2:) are fixed in the filemrkln01.ini. The details areexplained in the box titled ‘SoftwareOperation’.

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6310/2000 Elektor Electronics

Figure 4. PCB layout and component overlay.

(C) ELEKTOR 000066-1

B1C1C2

C3

C4

C5C6

C7

C8

C9

C10C11

CON1

D1 D2

D3

IC1

IC2

IC3

IC4

J1

JP1

K1 K2

K3K4

R1R2

R3

R4

R5

R6

R7

R8

R9

R10

R11

S1 S2T1

T2

T3

000066-1 STOP GO

~

B R

(C) ELEKTOR 000066-1

COMPONENTS LIST

Resistors:R1,R2 = 560ΩR3,R4 = 1Ω5 5 wattsR5,R6 = 10kΩR7 = 2kΩ2R8 = 4x10kΩ SIL9 array, 1 commonR9 = 22kΩR10 = 47kΩR11 = 8x10kΩ SIL9 array, 1 common

Capacitors:C1,C2 = 2200µF 40V radial

or C3 = 4700µF 35V axialC4 = 220nF MKTC5,C6 = 15nF MKTC7 = 10µF 16VC8 = 1nF MKTC9 = 10nFC10,C11 = 100nF ceramic

Semiconductors:B1 = KBPC601 (6A bridge, International

Rectifier)D1 = LED, red, 3 mmD2 = LED, green, 3 mmD3 = zener diode 5V6 400mWT1,T2,T3 = BC547BIC1 = L6203 (ST Microelectronics)IC2 = 4001IC3 = ULN2803AIC4 = MC145026 (Motorola)

Miscellaneous:S1,S2 = pushbutton with make contact (e.g.,

Diptronics DTS-6XX)CON1 = PCB mount 25-way sub-D plug

(male)JP1 = 3-way SIL pinheader with jumper, or

changeover switchHeatsinking material for IC1 (e.g. aluminium

bracket, min. thickness 2 mm)PCB, order code 000066-1 (see Readers

Services3,5”-inch floppy disk, Windows control

software, order code 996016-1

Optional:Power supply transformer, 15 V / 5 A, as an

alternative for the Märklin transformer

this one is slightly thicker and the link carriesthe entire output current.

The PCB offers the option to use two radial(upright) electrolytic filter capacitors (C1 andC2) or a single axial one. C1 and C2 may bothbe 2200 µF. This is sufficient, but larger ones (2× 3300 µF or 2 × 4700 µF) are permissible andwill also fit. A single axial electrolytic capac-itor (C3), as used in our prototype, is also apossibility. The operating voltage was delib-erately selected to be on the high side (35 Vminimum) to ensure that the circuit will sur-vive if the Märklin transformer is inadver-tently switched to direction-reversal.

CON1 is the connector with which the cir-cuit is connected to the parallel port of a PC.This may be mounted directly to the PCB.Take care that the force of plugging orunplugging the connector is not transferredto the solder connections. To ensure mechan-ical strength, there exist connectors withholes for mounting screws or a kind of barbfor soldering to the PCB. Either type fits. Ifyou prefer not to connect the circuit directlyto the PC, but position it closer to the railway,you can use a DB25 extension cable (avail-able for a small outlay in every computerstore). Alternatively, you can solder the wiresof such a cable directly into the circuit board.

S1 and S2 are switches with momentarymake contacts. The switches specified in theparts list fit directly in the PCB. However,other types may also be used if they are con-nected with a short length of wire.

A similar story applies to JP1, whichswitches the function on or off. Three pins ina row and a jumper are satisfactory, becausethe function is usually switched on. MiniaturePCB mount switches with changeover con-tacts also exist. Of course, an external switchattached with hook-up wire is also possible.

The tolerance of C8 is critical, because itdefines the timing. Use an MKT-type(Siemens or similar) capacitor.

The rectifier bridge (a very common typefrom International Rectifier) will, in the eventthat maximum output current is demanded,appreciate a small amount of heatsinking.Prior to fitting onto the board, insert an M3bolt in the mounting hole of the bridge (thescrew head is at the bottom of B1). B1 maynow be soldered onto the PCB. Finally, makea thermal link, using a right-angle section ofaluminium extrusion, to the right angleheatsink onto which IC4 is mounted (refer toFigure 5). The hole in the bottom of the cir-cuit board allows access so that the screwcan be tightened. Don’t put the screwthrough both the rectifier bridge and the PCBor you will run the risk that, when tighteningthe screw, the circuit board traces get peeledoff the board. Alternatively, you can mount B1

GENERALINTEREST

64 Elektor Electronics 10/2000

Figure 5. Construction of the thermal link for the bridge rectifier.

Tabel 1. Loco addresses and trainspeed/reversing codescontrol loco address Delta address A1-A4

1 2 X 0 0 02 6 0 X 0 03 8 X X 0 04 18 0 0 X 05 20 X 0 X 06 24 4 0 X X 07 26 X X X 08 54 0 0 0 X9 56 X 0 0 XA 60 3 0 X 0 XB 62 X X 0 XC 72 2 0 0 X XD 74 X 0 X XE 78 1 0 X X XF 80 X X X X

Data bit Coding ResultA5 0 function (lights) off

1 function (lights) onA6..A9 0 0 0 0 stop

1 0 0 0 reversing command0 1 0 0 speed level 11 1 0 0 speed level 2 (etc.) …1 1 1 1 … speed level 14

0 = logic zerolX = logic open

Naturally, any other 15 V transformer can alsobe utilised. From a safety point of view, westrongly discourage connecting the sec-ondary windings of different transformers inparallel.

First check the voltage across C1 and C2(or C3); this may be 20-25 V at the most. Thencheck the logic power supply, for example,between pin 8 and pin 16 of IC4. This must bebetween 4.8 V and 5.2 V; the exact value isnot critical. If everything is in order, then theentire circuit may be connected to the PC andthe tracks, using the connections B (brown,outside rails) and R (red, centre rail).

After switching on, the circuit will nor-mally be in stop-mode. Push the appropriatebutton to activate the run-mode. Voltage isnow applied to the tracks. In case of an over-load, the circuit will automatically switchitself to stop-mode. Install the soft-ware by copying the two required files(mrkln01.exe and mrkln01.ini) to a directoryof your choice (they must both be in the samedirectory). The file mrkln01.ini contains thedefinition specifying which printer port thecircuit is connected to. This file also containsthe text labels for the operating controls.After starting up the software, the screen willbe as shown in Figure 3. Operation with amouse is self-explanatory, for operation fromthe keyboard we refer you to the appropriatebox. If it doesn’t work, check first that thecorrect printer port (888 = LPT1:, 632 =LPT2:) is selected in the mrkln01.ini file.Another possibility is incorrect timing of theencoder. In this case you will need to tweakthe values of C8 or R9. Those who hungerfor more output power may connect terminalsR and B directly to the input of the EEDTs-booster. If the tracks are fed only via thebooster, then additional cooling for IC4 andB1 is not required. A few groundissues to keep an eye on, particularly if theEEDTs-booster is connected. Mains earth isconnected through the PC to ground of thecircuit (negative terminals of C1, C2, C3).Because the output is a full bridge, output Bis NOT connected to mains earth. The R andB terminals of the EEDTs-booster must beallowed to float with respect to mains earthand this applies to the entire railway as well.When connecting (grounded) measuringequipment this has to be taken into account.

(000066-2)

directly to the heatsink and bend theconnecting leads in a right angle;they should be long enough.

IC4 is also mounted on theheatsink. Electrical isolation is notrequired (the metal part is connectedto the GND pin), but heat conductingpaste is required. Remember thatnone of the AC inputs (K1 and K2)are connected to ground.

Powering up

It is always exciting the see whetheror not the result of your industriousactivity transforms into smoke whenpowering up, especially when

‘power’ is involved.To be safe, it is always good prac-

tice to check the power supply volt-age first. Connect the circuit termi-nals K1 and K2 to a Märklin trans-former. By initially using the brownand red terminals (instead of the yel-low) you can make a cautious startwith a lower voltage.

The standard Märklin transformeris rated 30 VA. This is too small todeliver the maximum output currentof 3.5 A. However, actual usage willindicate if it is sufficient for your nor-mal use. The 50 VA ‘lighting trans-former’, with its fixed 16 V outputvoltage, may be more appropriate.

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6510/2000 Elektor Electronics

Software OperationKey combinationsThe program may be operated without using the mouse(applicable to mrkln01.exe V1.01)

Key (or combination) result<TAB> next loco control<SHIFT><TAB> previous loco control<1>-<9>,<A>-<F> direct selection of relevant loco control= repeat same key, train reverse

<CTRL><1>-<9>, <A>-<F> enable / disable relevant loco controlcursor up/down keys increase / reduce speed<0> speed 0, immediately on selected control<space> speed 0, immediately on all controls

(software emergency stop)<ALT><F4> quit program

Settings in file mrkln01.iniThe file mrkln01.ini (residing in the same directory as mrkln01.exe) containsinformation on currently used loco controllers (loco addresses), the printer portused, and the text strings inside the control buttons. Comment not included inthe .ini file itself is shown to the right.

[PARAMETER]CH1=0 0 = control not in useCH2=0CH3=0CH4=0CH5=0CH6=1 1 = control in useCH7=0CH8=0CH9=0CH10=0CH11=0CH12=1CH13=0CH14=0CH15=0ADDRESS=888 printer port; 888 = LPT1:, 632 = LPT2USE_AD=0 not usedSAVE_SETTING_TEXT=save settings text may be editedEXIT_TEXT=close to requirementSTOP_TEXT=stop all trains