# R2LC/R4LC Serial Port Buffer



## gunrunnerjohn (Nov 10, 2010)

With some TMCC upgrades, or even adding the Cruise Commander M to some locomotives, the serial data stream from the R2LC (or R4LC) gets overloaded and doesn't properly trigger either the motor control or the sound. I previously created a transistor buffer to correct this, but that doesn't always work. I set about finding out why it would sometimes fail, turns out it's the amplitude of the serial data, not the actual drive impedance. Since the transistor buffer was an emitter follower, it didn't increase the amplitude, which turns out to be the bigger issue. So, back to the drawing board.

I came up with this. It's a 2x amplifier using an op-amp that also provides at least 20ma of drive if needed.










Simple circuit, any caveman could do it.








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It accomplishes the goal, with a 2.5v P-P input signal, it outputs a rail-to-rail 5v P-P replica. Even adding a 470 ohm resistor load didn't phase it, the amplitude remains almost exactly the same. The 5VDC required can be stolen from the R2LC 5V lines on that board's pin-19 & pin-20.










The actual board is pretty tiny, .3" x .4", but I'm thinking I may actually slightly increase the size to make it easier to assemble.



















I believe this will for once and for all put to bed the issues occasionally encountered with loading on the R2LC serial data signal.


Given that the tiny components on the prototype are probably out of practicability for many folks, this is probably what the one for the masses to build would look like. In order for it not to get really large, I did keep SMT for the op-amp, but it's a SOIC with leads spaced a 1.27mm, they're pretty easy to solder. This one is still reasonably small for O-gauge use, .4" x .7" and about 3/8" tall. Functionally, it would be identical to the SMT version I posted above. It also is the same schematic as the SMT version, just the device footprints are different.

Now that I know the design works, I'll get some of these boards and build up one or two of the "consumer" grade models, and perhaps package up a kit of parts for folks to use.


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## Aard D'Vaark (Aug 1, 2019)

So I -assume- Hennings will be seeing these in a while ??

ps .. so where did the signature go to ??


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## gunrunnerjohn (Nov 10, 2010)

Actually, I doubt these will make it to a production product, not enough demand to get a quantity built. That's why I laid out a thru-hole version, I'll probably get some boards made and have components in stock so if someone wants these, they can build it from a kit. It's more challenging to build the all SMT version, the resistors and caps are truly tiny, and the op-amp has pretty closely spaced leads. After building a couple of those, I decided that most folks are going to have issues with the soldering.


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## Aard D'Vaark (Aug 1, 2019)

good enough, a 'big' kit it is , lol


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## gunrunnerjohn (Nov 10, 2010)

Yep, the kits would probably have to be shipped by motor freight, probably too big for standard shipping services.


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