The motherboard concept has been discussed elsewhere. It is intended to simplify installation of an S-CAB battery powered, radio-controlled system in small-scale locos. The purpose of this document is to report its application in a HO-scale Atlas-Kato GP-7. For brevity, we’ll skip the details and go directly to the result.

​This loco is ready to roll. All it needs is a body shell, which slides over the installation onto the chassis without much effort. It’s shown naked and annotated as an introduction to use of an S-CAB motherboard. 

​For this installation, all components are mounted on the motherboard, including loco’s lights. This arrangement is a convenient characteristic of a GP-7 and other high-nose diesels. Consequently, no wiring connects to the body shell. 

​The motherboard assembly is completed and tested on the workbench. It is then mounted on the loco chassis and permanent connections made to motor and truck wheel wipers. Additional tests are completed before replacing the body shell. For example, does the forward direction command cause forward motion and operate the headlight.

​No need to replace the body shell for a work-bench test-run. No need for track or power. Just turn on BPS battery power using a magnet wand and the loco (with or without shell) is ready to roll but be careful it does not end up on the floor.

​Aside from running off bench-top, this is a low-risk test. The motherboard was tested before making chassis connections and there is no external power source to cause trouble.

​A track test is higher risk. First, try it with no track power, which is not much different from a bench test, except the loco can be operated more thoroughly if its battery has enough charge. To test battery charging, turn on track power. This is the step where serious trouble can occur, which is a good reason to do it without the body shell. Common mistakes are forgetting to isolate motor terminals from loco frame, bad choice of power supply, shorted truck wipers, etc. 

​A green LED on BPS-v4 indicates if battery is charging. It's not obvious because the green LED is on the underside when BPS is mounted on the motherboard. The BPS is oriented so that reed switches are topside where they can be activated by a magnet wand. Another good test is turn off battery power (blue LED off) before turning on track power. BPS starts automatically when track power turns on.

​This is a critical step in any installation. Will the lid close? The motherboard helps by enabling test fitting more easily throughout the project. Wiring clutter is eliminated, components don’t rearrange themselves by accident, and tight spots inside body shell can often be trimmed. The most difficult issue is headroom. Is there enough clearance above the loco’s mechanism for the motherboard assembly? This question must be addressed when planning an installation.

Good news. The lid fits.

​The following components are mounted on the motherboard:

• BPS-v4: Provides 11 volt power supply from 3.7 volt LiPo battery. Includes battery charging from track or other external power
• Battery: A 1P-420 LiPo cell with integrated 6 amp fuse
• Sound Decoder: A SoundTraxx TSU-1100 sound decoder
• Radio receiver: An S-CAB radio receiver integrated with decoder
• Speaker: A 1 watt mini-cube
• Head and rear lights: Surface mount LEDs with 4700 ohm resistor

With no components attached to the body shell, there is no wiring to prevent its removal. Easy preparation for painting this model is an immediate benefit. 

​Battery replacement: The battery is best secured to the motherboard by a removable method that does not leave a mess to clean up. Kapton tape is good and leaves no residue. Double sided adhesive tape is not recommended. When freed, lift the battery and lever its plug from BPS socket with tweezers or needle nose pliers. Since the battery leads are deliberately short (about ¾”) and the plug is tight, pulling by the battery may remove the battery and leave its plug stuck in BPS socket.

​If the loco’s mechanism needs maintenance, unsolder motor and truck pickup connections from the motherboard. If glued in place, there are 2 locations to pry loose and the assembly lifts off as a unit. If held in place by Kapton tape (author’s preference), cut and/or peel off the tape to remove the assembly.

​This installation was performed by John Lloyd in four identical models for a Canadian customer. The motherboard was designed to fit the GP-7 as well as a popular selection of North American diesel-electric locomotives. EMD GP, F and E class locos are good candidates. As we gain experience, the motherboard may be provided as basic, partial and fully assembled versions.

MB-GP-v2 fits a broad selection of HO scale, US style diesel-electric locos; EMD GP series, for example, often called "Geeps". It measures 0.67” wide, 5.75” long (17 x 146 mm), which is small enough for a GP-7 installation, yet large enough to accommodate a complete battery powered, radio controlled, ready-to-run system using commercially available DCC sound or non-sound decoders.

In addition to mounting BPS battery power supply, battery, radio receiver and decoder, the board ​includes terminals for external connections; track power pick-up, motor, LED lights and speaker. Depending on loco, lights and speaker can be mounted on the motherboard, which eliminates wiring connected to loco’s body shell and requires only 4 connections (power pick-up and motor) to loco chassis/frame. 

​A 9-pin JST connector is provided for decoders with NMRA-compliant socket. No wiring harness is required. Decoders without a compliant JST socket require soldered connections and careful reference to decoder documentation. The BPS-v4 battery power supply can be mounted directly on motherboard (as in photo) or with a wired connection.

Add a decoder: This motherboard version requires either a SoundTraxx or NCE decoder with integrated S-CAB receiver. As shown in photo, no wiring is required for a SoundTraxx TSU-2200 decoder; just plug it in.

Plug in a battery: Depending on loco's space, 420, 800 or 1000 mAh batteries fit on the motherboard. If clearance between top of motor and roof interior permits, a 2P battery may be possible. However, 1P and 2P-420 are the only choices for HO scale, narrow-hooded GPs. The following photo includes a 1P-420 battery. Note the 3/4” battery leads, which will be default choice for batteries included with a motherboard installation.

For HO models, application depends on the model’s prototype and its manufacturer. S-scale modelers should find the motherboard an easy fit. Fully assembled with a 1P-420 battery, the motherboard requires 0.44” (11.2 mm) headroom. Allow 12 mm (approx 0.5") clearance if using a 2P-420 battery. 

Models with limited headroom, motor mounted high on the chassis, for example, are not good candidates for this motherboard. Small yard switchers (SW series) in HO scale can also be eliminated as candidates. This still leaves a large selection of diesel prototypes for which this motherboard is applicable. However, creating unobstructed space can be a challenge. Factory installed circuit boards, over-sized metal castings, light-pipes, existing wiring and other clutter may have to be removed.

​This example used version 1 of the motherboard. ​No modification of loco chassis was required. The shell required some work. Light pipes were removed and interior of body shell decluttered. Lights and speaker were mounted on motherboard.

​The lens portion of light pipes was retained and glued into body shell.

​Components used:  TSU-1100 decoder, S-CAB radio receiver, 1P-420 battery, BPS battery power supply, mini-cube speaker, LED headlight and rear light.

NCE and SoundTraxx manufacture decoders referenced in this blog. Their proprietary rights, including product names, is acknowledged. 

​Effective October 31, 2018, we introduce two new battery sizes and phase out BPS-300 and BPS-500 batteries. 

​BPS-420
​This is the battery of choice for HO-scale narrow-hooded diesels (GP series, for example). It replaces BPS-300 and BPS-500 batteries which will be discontinued.

​
​BPS-1000
This adds a larger cell to the BPS battery range. It is intended for use in steam loco tenders and in S-scale diesel locos.

BPS-850
There is no change here. This  is the most popular BPS battery used for HO-scale full-width diesels (E and F series, for example) most steam loco tenders and all larger scales (On30, On3, S). The new BPS-1000 is expected to replace it in some of these applications.

​The 300 mAh cell is no longer available from manufacturer and is replaced by higher capacity 420 mAh cell which is same width and half an inch longer. The BPS-500 cell, which has been sourced from a different supplier, is replaced by the 420 so that all LiPo cells can be purchased from the same manufacturer.

Each cell size will be available as 1P and 2P back-to-back assemblies. Integrated fuse protection is standardized at 6 amps to be compatible with latest version of Battery Power Supply (BPS-v4) which requires up to 4 amps battery current at full output.
"P" indicates cells are connected in parallel. Voltage is same as single cell (nominally 3.7 volts) and capacity adds so a 2P-420 battery provides 840 mAh.
25C indicates cell is rated for high discharge; specifically, 25 times cell capacity. That is, 25 x 420 mA equals 10.5 amps from one cell and 21 amps from 2 cells in parallel. This should remove any doubts regarding the importance of embedded fuse protection. 

When a back-to-back arrangement is too thick for some applications, cells can be wired with an arbitrary length of wire between them. With the arrangement illustrated (right) cells share one fuse as is done with a back-to-back assembly. However, this leaves one cell with external wiring and no internal protection. Although the risk is low, this practice will be discontinued. 

In future, separated 2P arrangements will use two 1P batteries (each with fuse) connected in parallel. One precaution with this arrangement is make sure cell voltages are approximately equal before connecting. The equalizing current surge created by a large voltage difference may be sufficient to blow one or both fuses. Be sure to connect positive to positive.

​​This fourth generation BPS power supply is intended for on-board battery power installations requiring up to 1 amp from a package small enough for installation in HO, On3, On30 models. It's also a good choice for S scale models with efficient can-type motors.

Batteries for previous versions of BPS included a 3.5 amp fuse and used insulation displacement battery plugs.
At 1 amp load, BPS v4 consumes up to 4 amps from the battery, which requires a 6 amp battery fuse, heavier leads and a crimp-style plug.

The protection block in the above diagram protects against three abnormal conditions.

• Excessive input voltage: Turns off input when voltage exceeds 20 volts and restarts at 18.5 volts. Since protection is virtually instantaneous, voltage spikes, peaks, as well as sustained conditions are controlled. 
• Overload: Rectifier output is limited to 1.2 amps, which is comfortably within the 2 amp rating of the rectifier diodes.
• Short-circuit: Fast trip (less than 1/4 msec.) occurs at 2 amps.

If a locomotive or traction model needs on-board DC power, BPS can do the job so long as demand does not exceed 1 amp. Temporary overloads up to 1.2 amps are okay. Standard output voltage is 11 volts, but can be customized within a range from 9 to 12 volts.

​Introduced 1987-89, these models, which were manufactured by Kato and sold by Atlas, have excellent mechanisms and are great candidates for S-CAB battery power and radio control. The cast metal frame is heavy enough for traction, but leaves space above the motor for electronics and battery. With some careful trimming of the body shell, this space can be extended the full length of the loco. The completed installation (photo left) shows no evidence of any modification to the loco.

​This description illustrates use of a circuit board (sometimes called a "motherboard") to simplify installation and complete as much work and testing as possible on the workbench. Wiring within the loco is minimized by mounting front and rear lights on the components circuit board. As a result, there are no wires connected to the body shell, which can be removed and replaced without restriction. The project involves modification of the loco body and frame, assembly and test of two subsystems and finally, installation.

​Traces on the loco interface circuit board are used for rail pickup connections to front and rear trucks. If convenient, motor connections can also be to this board, which makes a total of 6 wires (2 to each truck and 2 to the motor) to be disconnected should it ever be necessary to remove motor or trucks. 

​The component board sits on top of the interface board and is held in place by electrical tape.

Since LED lights are mounted on the components board, there are no connections to the loco body, which simply slips into place to complete the installation.

I occasionally hear from an S-CAB user (Michael) who models On3. Since he's an innovative guy, as well as a skilled modeler, his feedback is always interesting. His latest effort is an S-CAB radio and BPS battery power supply installation for a Climax loco.

Here are Michael's comments:
​
Thought you would like to see my latest car (trailing) for the battery and BPS electronics board. The decoder/wireless/speaker are in the Climax but I need a car for the battery etc.

You will note the two pipes (lower left with the chain over the pipes) – these are for power to the loco – there are two small plugs on the loco on the same side, so thin wires will do the connection.

Also, on the top rear of the tank are two small pipes – one higher than the other – these are the recharge points.  Just plug in the power supply and the recharge will start.  The tank is not firmly secured on the flat car just sort of held in position with the chains.  The barrels etc are glued down.  The chain (not glued) at the back is part of the chain keeping the tank in position.

I have two magnetic reed switches (the ones you supplied) glued up at the top inside both ends of the tank. So I just wipe with the magnetic wand for start and stop. 

In the background is another battery car – the tank.

This is a complete battery power, radio control, sound installation in a Bachmann On30 Forney. The entire system fits under the coal load with the battery barely visible where it protrudes on the floor of the driver's cab.

Since this model already has a factory-installed non-sound decoder, the hope is that installation can be simplified by use of existing wiring to motor, headlight and driver wheel rail pick-ups. It's also convenient that the fuel bunker is a separate molding attached to the frame by 4 screws.

Begin by clearing the clutter and planning efficient use of available space. Laying the battery flat on the loco frame and using space in the cab accommodates a 500 mAh battery. Obviously, the speaker must be mounted elsewhere. Internal space within the fuel bunker is 40 mm long, 40 mm wide and 22 mm high. Since the battery is approx. 6 mm thick, the challenge is to fit a sound decoder, S-CAB radio receiver, BPS battery power supply and speaker in the remaining 16 mm above the battery. Several arrangements are possible, but I eventually decided on a major rework of the BPS circuit board to make it a "middle layer", with radio receiver and speaker mounted at the top of the assembly. This gives good speaker acoustics and keeps the radio antenna clear of metal surfaces and wire tangles.

BPS has 2 standard arrangements. The long configuration can be cut in half and the 2 halves mounted back to back to form a short configuration.

BPS-L
This is the standard, one-sided BPS circuit board 

​At this point we have an installation plan and a modified BPS circuit board. Let's determine if everything fits.

What to do with a tangle of wires is often considered too late in an installation. For this model, the entire installation must fit a space measuring 1.575" square (40 mm) and 0.79" high (20 mm). The planning process included component placement as well as arranging wires to minimize both quantity and length while still being able to make the required connections. The final arrangement is illustrated below. There are 3 unused decoder function outputs (green, yellow and brown wires) where leads have been shortened, but are still accessible for (unlikely) future use.

The availability of new and upgraded components has initiated a major update of the website. New versions of Home, Radio Control and Battery Power pages are already online and others will be published as editing progresses. Here are a few areas with significant new information.

New versions of S-CAB radio receiver (LXR) and Battery Power Supply (BPS) are available

LXR: The new receiver board is designed for convenient integration with new decoders. Another design change involves antenna placement. After several years experience with various antenna orientations, the new LXR standardizes cross-wise mounting as shown in the photo. Vertical mounting is still available by special order, but horizontal (lengthwise) is discontinued, since it offers no performance advantage and increases overall length of the receiver.

BPS: A complete redesign of the Battery Power Supply provides major improvements:

• Size reduction: It's not dramatic, but decreased width is very helpful in narrow hooded diesels (GPs).
• Maximum output is increased from 500 to 750 mA.
• Integrated battery bypass operates loco from track power when available, and provides automatic, instantaneous battery backup. Deadrail operation or isolated battery power is optional.
• Battery under-voltage protection has been improved.

New decoders, which simplify S-CAB installation, have been selected as standard products for S-CAB systems. 

• NCE D13DRJ:  Combines a non-sound decoder with S-CAB compatible receiver on the same circuit board.
• SoundTraxx ECO-100:  A 1 amp, 4 function sound decoder in a very small package that integrates nicely with S-CAB receiver

NCE recently released D13DRJ, a new version of its popular D13 series HO decoders, which is intended for use in battery powered, radio controlled models. "DR" is derived from "direct radio" and "J" means it has an NMRA standard 9-pin JST socket and wire harness. The radio is compatible with S-CAB and battery power can be a BPS battery power supply or a series-connected, multi-cell (typically 11.1 or 14.4 volt) battery pack. One side of the circuit board is the decoder, which has been simplified and reduced in size by removing rectifiers that are required for DCC track input, but unnecessary with battery power. A Linx Technologies radio receiver, mounted on the second side of the board, provides DCC commands directly to the decoder microprocessor; the same method as used by S-CAB. The remaining radio-side components form an electronic on/off battery switch. The blue wire is an antenna. (A 3 inch length of wire makes a simple antenna.)

The decoder is "plug and play" when used with S-CAB radio and a battery pack. However, decoder red and black leads are no longer DCC track input. Red is battery positive and black is battery negative. Correct polarity is critical.

There a two ways to eliminate the confusion of duplicate switches. Either disable the electronic switch by putting it in a permanently 'on-state' when there is battery power, or remove the switch and replace it with a wire connection.

I'll illustrate removal for this blog. It's simple, but not reversible. All components except the radio are removed and the chip that forms the electronic switch is replaced with a wire.

The always-on modification suggested by NCE is slightly more complicated, but is a reversible procedure.

S-CAB users can order the D13DRJ decoder now at the same price as an S-CAB-compatible D13SR or D13SRJ. I'll add it to the price list page once I decide whether or not it replaces D13SRJs. It's a neater, smaller, easier to install solution for users who prefer non-sound decoders or are working with restricted space in small locos. 

NCE decoders sold by me for S-CAB use now include a firmware version that supports use of function F5 for turning off battery power and eliminates the requirement to use a separate switch or sensor for this purpose. This is part of an effort to "standardize" F5 as a battery-off command. However a switch can still be used if that is the user's preference.

I thank NCE for cooperation during this development and willingness to bring what may remain a niche product to market. As always, NCE's proprietary rights are acknowledged. 

Two new Walthers ALCO DL-109 HO-scale locos will be converted to S-CAB radio control and battery power for operation as a permanent consist with number 759 as the lead loco (the A unit) and 749 (B unit) running backwards. An identical set of components will be installed in each loco and the 2-loco consist will be operated by giving each loco the same decoder address. 

There are five components to install, three of which significantly affect space requirements. The BPS circuit board is fixed and the TSU-1000 decoder is required to safely handle power requirements of this heavy loco. This leaves battery, radio receiver and speaker for which we have options.

Here are a few considerations that led to the final arrangement illustrated below.

• For good radio reception, avoid placing the radio antenna against a metal surface.
• The BPS turn-on sensor must be outside the range of magnetic fields created by motor and speaker.
• The speaker should be located so that the motor magnet does not interfere with speaker performance.

The radio receiver is mounted at front of loco and part of the frame is cut away for two reasons:

1. To accommodate the speaker
2. To create some air-space between antenna and metal frame.

By mounting the receiver above the speaker, it is almost out of sight above cab windows.

The speaker is glued to a styrene plastic mounting that is attached to the frame with screws, which allows assembly to be removed for access to drive shaft and front truck. To complete this sub-assembly, the radio receiver will be mounted on back of speaker enclosure with adhesive or double sided sticky tape.

The BPS is installed at the rear of the loco where it sits in a plastic mounting attached to the loco frame with screws so that the rear truck is accessible. By using 1/8" styrene, the two largest BPS components (inductors) are almost flush with the lower surface of the plastic mounting. There is also a spacer below the mounting so that the BPS magnet sensor is positioned close to the loco roof.
This arrangement leaves space above the motor for battery and decoder, neither of which is sensitive to the motor's magnet.

Eliminating battery socket and direct wiring the battery connection is another space saver. Wire both battery and BPS with leads an inch or more in length, then solder and insulate to join leads as the last step in making the connection. This reduces the risk of accidental short-circuit while working with a charged battery and also provides extra wire to simplify battery replacement, should this become necessary.

There is nothing quite as frustrating as completing, testing and tidying up wiring, only to discover the loco's body shell will not fit with everything installed. To avoid this disappointment, it's best to perform several trial fits of the shell with various combinations of components temporarily held in place with electrical or Kapton tape.

As mentioned earlier, locos will be coupled back-to-back to form a more or less permanent consist. Since the locos are mechanically identical and their speed characteristics are similar in each direction, no speed curve matching is required and respective motor control CVs will have identical values. Configuring function outputs is easy, since a headlight (function F0) is the only loco feature to be controlled. One other function output, F5, will be used to turn off battery power.

Assigning the same decoder address to each loco, avoids the need for a separate consist address. Each loco's decoder, having the same address, will respond to a radio command for that address. The only reason to set up a consist address is a requirement for each loco to respond differently to the same command. This also applies to CV programming commands and is the reason loco 749's motor was connected with polarity reversed; it runs backwards in response to a forward command. A similar result could be achieved using CV29, but this requires the decoders to have different values for CV29. Except for the bell (which is set to zero volume in 749), both locos have identical values for all CVs.

With radio control, no programming track is required and, once the decoders have the same address, CVs are programmed simultaneously for both locos, using either operation or service mode programming. To set a CV value (for example, the bell volume) for only one of the locos, just turn off the other's battery power. If using service mode programming, be sure to turn off power for all other decoders to prevent them from responding to a globally addressed command.

To separate the consist and operate as separate locos, the S-CAB throttle can be used to change the address for one of the locos. If desired, CV29 can be programmed (using the S-CAB throttle) to reverse motor polarity of loco 749.

The consist is operated in the same manner as a single loco. Momentum settings should be the same for each loco with values that match acceleration and braking somewhat realistically with diesel engine exhaust sound. This can be done by using the S-CAB throttle to adjust the values of CVs 3 and 4.

Except for one operating scenario, decoder default settings operate headlights automatically. F0 toggles headlights on and off. With F0 on and loco 759 moving forward, its headlight will be on and 749's will be off. However, if the consist is backing a train into a station or siding, 749 will be shining its headlight onto the end of the nearest car. Operation will be a little more realistic if F0 is used to turn off headlights.

Battery power requires a method to turn batteries on and off. If the consist is on powered track, turn-on is automatic when BPS detects track voltage. If track is not powered, a magnet held close to the on-board sensor turns the battery on manually. With some wired connections between locos, both could be turned on using one loco's sensor. However, it's simpler to just turn on each loco separately (so long as we remember to do it).

If there is no track power, a decoder F5 command turns off batteries of both locos (since their decoders have the same address). Function commands toggle decoder outputs on and off. It may not seem intuitive, but turning F5 on, turns battery power off. However, the presence of track voltage overrides decoder F5 output and keeps battery power on. This can be the basis of a useful procedure as follows.