# Reply to "MTH Engine Gear Ratios"

Step 1: a fixed distance

I used RR-Track when I designed my layout. RR-Track has the ability to add up the length of a selected route. I picked my longest circular route and RR-Track told me that it was 1.5884 Smiles. Ellison fans will know that this was his term for a scale mile. In O gauge, it's 110 feet or 1320 inches.

Starting with the diesel's wheel diameter at 22.6 mm, 24 stripes on the tach strip and a 10.5 gear ratio, math and geometry tell us that we have 472.225 revolutions of the driving wheel to cover 1 Smile. That also works out to 119000.8 tach stripes per Smile.

My plan was to use that 1.5884 Smile distance as a standard length to try to understand the DTO and DOD commands.

Step 2: Collect the data

For several years, I've been studying the commands and responses between the remote and the engine. I learned a long time ago how to build a radio snooper to decode the commands and responses. Both the DTO and DOD commands elicit two responses from the engine, the first is a 16 bit number, the second is a 32 bit number. These numbers are read out from the RAM memory in the engine. The first number is always the same for any particular engine and is the same for both the DTO and DOD responses from that engine. The second number increases as the engine runs and does not change when the engine is stopped. The values returned for the DTO and DOD commands are very different but both increase as the engine runs.

My working assumption is that the 16 bit number somehow incorporates the gear ratio and wheel diameter. For the diesel that I'm working with, the value is 21637. In a lot of my documentation, I call this the Scale Factor.

Then the 32 bit number represents the trip odometer (for DTO) and the odometer (for DOD). The remote takes these two values and displays the Smiles on the display.

More to follow.

OGR Publishing, Inc., 1310 Eastside Centre Ct, Suite 6, Mountain Home, AR 72653
330-757-3020

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