Question about speed-steps, stops 28/128

[passenger1955]

I have questions about speed steps and stop packets. In NMRA S9.2 lines 48-60 it discusses 28 step packet structures. I notice the first 4 packets (in order) are 2 stop commands (00000 and 10000) and 2 emergency stops (00001 and 10001). I take it there are 2 of each because of direction, but I'm a little confused because I would think the Direction bit would handle that. Could someone offer a little clarity on what is going on there?

Secondly, I'm trying to understand how these get handled in 128 step speed commands. Assuming this is all handled in a separate byte, do the first 4 lowest values of that byte correspond to 2 stops and 2 emergency stops (leaving 252 more values for speed in 2 directions?)  Or do the first 2 lowest values in the byte correspond to stop and emergency stop, leaving 254 more values for speed in 2 directions?

Any help would be appreciated.

 

 

[rrinker]

 Looks under line 45, where it explains Byte 2. that bit in the chart of steps is the one labeled C - the one to the left of that is D, the direction bit. That paragraph explaining byte 2 explains that bit C is for backward compatibility, in this case with 14 step mode, where it controls the headlight function. This is why if you set CV29 on the decoder for 14 speed steps but your system sends it 28, the headlight will alternately go on and off with each speed step - bit C is the least significant bit of the speed, so Step 1 it's on, Step 2 it's off, Step 3 it's on, etc. And that's why there are 2 E-stops, one with headlight on (bit C is 1) and one with headlight off (bit C is 0).

 The rest is all handled with extended packets, 9.2.1 

                                   --Randy

[passenger1955]

Thanks Randy. I'll read up on that section.