Wayne Roderick, 3rd Division, PNR, NMRA (life)

05/29/98 rev 01/16/07

The Teton Short Line uses a system of walk-around cabs that plug into a multiplexed (MUX) "party-line" at any of many jacks scattered around the railroad. Train control commands are put onto the MUX line while digital voltage, train numbers, official time, fault alerts etc. are read from the MUX line by each cab. Each hand-held cab becomes your total communication link with your train while having the freedom of movement exceeded only wireless or radio control, but with the added advantage of feedback instrumentation

The highly successful walkaround cab system that we describe here is one of two major sub-systems of the the Teton Short Line CONTROL SYSTEM. The other sub-system was the Automatic Cab Control(ACC) that keeps your cab connected to your train. ACC has been replaced with Digital Command Control (DCC), but the walkaround cabs remain the same. The TSL MUX system is not described in sufficient detail to copy it, partially because the technology is twenty years old and most of the drawings are still in original pencil sketch form. We offer it, like the other TSL systems to give you ideas and stimulate your thinking.


The Teton Short Line MUX system was conceived about 1978 and constructed as a stand-alone control system that transmitted the engineers commands over a plug-in data line to circuitry that ultimately generated the desired electrical waveform and polarity for the loco motor. It was the job of the other sub-system, the ACC to keep the loco connected to it's cab. It was hand-wired, using TTL logic on 12 Radio shack plug-in perf' boards (cards) and assembled in a circuit card frame that we'll refer to as "CENTRAL". The walk-around cabs connect via tip-ring-sleeve telephone plugs and jacks scattered about the railroad.

MUX system, Original interface

The data line is time-shared among the various devices, four cabs, dispatchers panel, official RR time clock, and a number of circuit cards in the CENTRAL card frame that feeds or reads data to and from the cabs. We did not use any particular communications protocal but just typical technology of the time. A sync' pulse starts a counter in all devices and the count is then advanced with clock pulses on the system, so all devices are "on-time". Each device that sends data has an assigned time slot. Any device on the line can read the data, regardless of where it originated. A cab sends in it's assigned time slot and reads data intended for it in a particular time slot, it can read data intended for everyone, such as the official railroad time. This system is called a "synchronous multiplex" sytem


Experience in the work place has taught us that there are very few successful plug and socket systems that are easy to use and reliable. The ultimate seems to be the tip-ring-sleeve jack and plugs used in the telephone industry and developed to a fine art. We use these- don't even consider the cheap ones sold for the audio world. Buy the real thing. The problem we face with this rugged choice is the limit of three wires and we've got to carry Power, Ground, Data, Clock and Sync' pulses to the cab on a light weight coil cord. Now the fun starts. Today, we could use battery power, CMOS technology, pics and asynchronous communications, but we aren't building it today- it's 1978 and TTL circuitry is power hungry! Today, it still makes a good hand warmer on cold winter nights and it's not broken, so why fix it.

One wire (sleeve) is ground. Another wire (ring) carries the DC power that has been amplitude modulated with the clock signal. The third wire (tip) carries the 5 volt TTL data line with a 10 volt sync pulse riding on it. This is easily seen in a the handheld cab schematic.


The handheld cab is described from the operators viewpoint in the CONTROL SYSTEM page Compare the cab panel layout with the schematic and you'll see that we had a fun time stuffing it all in that little box. Today you wouldn't even consider such a task with the wonderful chips we now have, but just in case you're a little short of pixels, you can download the same schematic in four GIFS that can be pieced together for higher resolution. ZIP file.

DATA SCHEME AND ERROR CHECKING. Being a synchronous system, no preambles or identification is needed because the data source is identified by it's timing relative to the sync pulse. We use only 256 time slots or 32-8 bit words - Doesn't sound like a lot, but we use them efficiently. Running at 10kHz, Data is updated 40 times per second. Obviously we could handle much more, but the TSL needs were easily met. Only four bits are needed for a digital charactor, so sixteen bits (2 words) carry the hour and minute data for the "official railroad time" A cab can issue up to 14 command (bits 1-14) plus two more bits used for framing (2 words*4 cabs=8 words). The framing bits, ,(0) and (15), from a cab must be received to insure that the cab has not been plugged in or removed during it's transmission AND only one of the two bits (8 and 9) that designate train direction must be received to verify that the line was not a shorted during the transmission. A transient short (logic ONE) might can occur as the cab is plugged in or removed. This is all the error checking that we have found necessary. Unique digital data sent back to each cab uses up another 1-1/2 words per cab

The fourteen CAB command assignments have varied somewhat but the most important are: Accelerate, Train airbrake, Engine dynamic brake, Dirction E/W/neutral, Lesser functions have operated a whistle, select digital data (train/engine number), call the dispatcher and initiate a cab/block assignment for ACC, select volts/speed or amperage for digital display.

The DISPATCHERs panel, abandoned and replaced with a CRT, read the data line and displayed all the digital data. His panel also had the ACC and Dynatrol initiate switchs now replaced by a computer keyboard. He read and displayed most everything with 7 segement LED displays, but only used up sixteen bits (2 words) to send commands.

We still have 64 bits (8 words) unassigned and we're updating 40 times per second. Got lots of capacity left.

Most of the CENTRAL encoding and decoding cards and functions originally on hand wired prototype cards, have been replaced by the computer. The only remaining cards are a Master Timer, a decoder card and the throttle interface(s).


11/28/04 Much information explaining the analog throttle interface has become so obsolete that I feel it has no further value to others and is eliminated. Today, as I have done, you simply go with DCC. Sadly, there is NO STANDARD for a cab data system as there is for DCC on the track. We continue to use and enjoy the Teton Short Lines cab system that is now connected via the computer system to EasyDCC.


All the above hardware was running just great when I got acquainted with Abbott Lahti of Dynatrol in 1986. We done some brainstorming about mixing the Dynatrol Command Control system with computers and it led to this. Changes are shown in Red.

MUX system, With Dyantrol

I wanted to explore the merits of the Dynatrol system to gradually replace or augment the ACC system of keeping your cab connected to your loco. Abbott had fixed us up with a DPS5 power supply, a DFC cab, a RL4 receiver, programming resistors and all the documentation. Initially, we just assigned one ACC cab to the Dynatrol system while I learned about it. I was very impressed with the engineering in the system. Abbott had used popular off-shelf components to build the system. Nothing was hidden in plastic or firmware but he did have his patents done up right. The open design assured me that Dynatrol would live a long life even if the sole source gave it up and that's a mighty important consideration for any sole source hardware. Over the next two years (87-88) I used several techniques to get the cab commands into the computer, which by now was a Heathkit CPM DOS mother board complete with a 5" disk drive that held 90k, but no keyboard or monitor. We built up four I/O cards that would generate the Dynatrol control frequencies and feed their output onto the Dynatrol signal wire. Acquired some more receivers and kept the original Dynatrol Direct Function Cab (DFC) in service too. At this point, we got into the fascinating world of simulation because having the computer sitting between the cab command and the command going to the loco, most anything you might imagine can be easily done in software. Acceleration control, Offset starting voltage, various response curves, multiple unit lashups et al. Most of the good things that Digital Command Control (DCC) has today, I was enjoying then.

It was a fascinating time, years ahead of most, but we gave it all up because I was alienating our Thursday visiting engineers. We've always enjoyed sharing the TSL with friends and their equipment. If you were wired to NMRA Standard S-9, as most equipment is right out of the box, and in good mechanical condition, you were welcome on the TSL. We went back to the friendly ACC. The Dynatrol hardware lanquished around until I committed to DCC and auctioned it off on E-bay.

DCC is the STANDARD (NMRA S-9.1&2). That means it's multisourced, will be around forever and get relatively inexpensive. It's not the best system in my judgement factors, but it is the standard. Hey- anybody know someone interested in 400 movies on Beta video tape ;-)

Along about 1998 when we were building up the new signal system , I decided it would be OK to let the computer be evident and besides I needed it handier for the signal developement work, so we plugged in a monitor and keyboard. The computer was now an IBM clone with MS-DOS. Changes are in red.

Over the next few years, the computer took over all systems. Today, the MUX/computer interface is a two-way device so that the computer reads everything on the data line and can put data on it at any time. The MUX throttle and brake commands will be passed onto EasyDCC. The computer keeps the MUX Fastclock, loco number and speed displays on the cabs updated.

Today, much has changed, ACC is gone, DCC is in, racks of circuit cards are gone, the old three-terminal detectors are gone! Common rail wiring is gone! but the MUX system remains as part of our standard.

I have adopted Bruce Chubbs CMRI RS485 bus system and protocal for INPUT/OUTPUT data as the Layout Control Bus (LCB). Getting the MUX system into this required a unique interface. The interface has memory chips that pass the information back and forth between the 10kb synchronous MUX system and the serial 19,400 kb CMRI bus.

The cabs remain essentially the same, They have stood the test of time and we like them. They're still not broke, so why fix 'em? The simple commands originating at the cab can be massaged by software before generating the DCC commands that go onto the locomotive(s). With the power of the computer, we really have very little use for many of the features built into the DCC decoders. We have virtually unlimited capacity using software simulation to make the experience of running trains feel even more real.

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