DIGITAL COMMAND CONTROL ON THE

TETON SHORT LINE (INTERFACE)

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

02/16/98 rev 01/15/07

12/30/02 update: Our DIY DCC has served the Teton Short Line well but our HO modular club has been using EasyDCC for several months now and we love it. Well designed and published in Model Railroader Feb-Jul 97, it has a great track record and is well documented in it's 180 page manual, making it a perfect candidate to upgrade the TSL. The upgrade will add four-digit addressing, two stationary cabs, accomodate visiting power without reprogramming and be familiar with visitors that use our club system.

I asked Santa for the EZ Command Station (CS) and he delivered VBG. The CS has RS232 interface that makes it a natural for computer control using the TSL cab system. It will replace the interface unit described below and should drive our boosters too. We'll keep you posted.

DCC INTERFACE DEVICE (DCCID): This device in conjunction with the TSL's embedded computer replaces the signal generating functions of a typical COMMAND STATION. The embedded computer, in addition to many other tasks, examines commands from the walkaround and stationary cabs, processes them in simulation software and then outputs the results to the DCCID via a periphial interface adapter (PIA), chip type 8255. The PIA resides along with other devices on on a custom wired board plugging into a slot on the 386 motherboard. The PIA is cabled to the DCCID with a 25 conductor ribbon cable. The computer language is plain vanilla QBASIC 4.5.


Schematic, DCC Interface

Unlike some commercial systems that have a variable length queue of commands and a dedicated processor to generate the data, we decided to optimize the system for our own needs. Our computer is busy with many tasks on the TSL and we prefer to not tie it up generating repetitious data. The DCCID has a RAM chip that records the computer commands for 21 loco's. The stored commands are continually scanned and output to the track via the power boosters. As each command takes a nominal 10msec, the most recent command for each loco is repeated several times per second. The number 21 could easily be increased, but it is more than adequate for the TSL's potential. The RAM is lightly used, holding only 128 bytes, but it greatly reduces the load on the embedded computer which only has to issue command changes. It can update the DCCIC several times per second in response to a cab command or its simulation software.

The basic signal generator is Mike Brandt's DCC-MB. It's simple, works well and I had plenty of TTL in the junk box.

My original design shown includes some features that I later abandoned because we learn. Programming with this homemade DCC evolved to using Digitrax PR1 device connected to the computers serial port and then to a homemade "improved" version of the PR1. Ultimately, it was all replaced with EasyDCC, but it was fun and challenging while it lasted. .

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