Plantraco DSP4-SC Receiver/ESC: Yet Another Hack

[Mr.RC-CAM]

It works!

The Plantraco DSP4-SC digital micro receiver has a built in 10A ESC. I was interested in using this doo-dad, but the Plantraco folks only offer it in the Futaba shift version. I fly JR.

But, I decided to buy one and convert it to the JR positive shift modulation mode. It arrived this week and I had time today to look at the little guy.

It uses a TA31136 FM IF detector IC and two 8-Pin PIC12C508 microcontroller IC's. One PIC windows and decodes the channels and the other is used to control the ESC's features. It is a really small receiver, that is for sure.

Well, after about one hours work I have a JR controlled micro Rx with built in ESC!

It has no bad tendencies and range is good. The hack required a tiny SOT-23 digital invertor (about the size of a pin head) and a resistor. It was added BEFORE the DC restore and filtering network, so I did not have to re-optimize the existing components.

I also had to move the ESC over to the correct JR throttle servo channel. Futaba uses channel 3, but JR uses channel 1. This bit of tricker was easy to solve. I lifted three pins on the ESC's PIC reordered them by jumping them to the decoder PIC. It all works well, at least on the bench.

I did notice one odd problem that may or may not be related to my mods. The throttle arming method requires that low throttle be held for at least one second before the motor will be enabled. This works for me too, but only if the servo pulse width is above 1.0mS. If it is set below 1.0mS then arming will not occur. It is an easy fix and merely requires a little Tx ATV adjustment. But it could cause confusion to other folks if it occurs on stock Rx's. My hunch is that the ESC PIC's error checking is involved. Anyone else notice this on their stock DSP-4C?

At this point my work looks a bit like a rats nest and I have to pretty up the installation. Sorry, but I never show my ugly engr stuff, so photos will have to wait until I finalize the installation. ;) I will post the mod if I like how it works in actual use.

I had time to make some measurements before and after I performed the shift mod. Jitter is about 5-6uS and servo step size is 5uS. This works out to 200 steps per full stick range (1.0mS - 2.0mS), which is about 7.5 bits of servo resolution. Not bad for a little 4 Mhz state machine driven PIC decoder.

I have yet to fly with it, but at this point I may very well have earned the privilege to claim that I am the first end-user with a JR compatible DSP4-SC. I hear that Plantraco will eventually offer one, so I will soon lose this prestigious placement. ;)

Regards,
Mr. RC-CAM

[BEC]

Hmmmmm..... interesting. I didn't think about the throttle-channel bit, but that will certainly complicate the production of the positive-shift version because there is nowhere near the apparent standardization of the thorttle channel as on negative shift equipment.

GWS uses channel 1 as does JR. Airtronics uses channel 3. My Multiplex uses 4.

I wonder how that other combo ESC/RX that Great Planes has handles this.....

[Mr.RC-CAM]

My Multiplex uses 4. I believe that you are in luck. It is my understanding that the Multiplex is not prejudice towards any fixed channels. If true, why not just re-assign the sticks in the model memory?

For sure, the lack of channel assignment standards makes integrated Rx/ESC designs a bit more complicated to market. It could be addressed via a stick enabled setup feature and a EEProm based PIC. Or silly jumpers.

While at it, the Rx's shift issue could be user selectable/programmable too. Incremental material cost should be mere pennies.

Regards,
Mr. RC-CAM

[Mr.RC-CAM]

Hack Summary Info:

This photo is of the top side. I had to lift two pins on the Motor Control PIC then jump them to the Channel decoder PIC. This reassigned throttle to Channel one, which is where the JR Tx expects it.

[Mr.RC-CAM]

On the bottom side the mod is a bit more tedious. The photos do not readily reveal that everything is very small.

I had to invert the output of the TA31136 FM detector IC. I used a tiny three terminal digital invertor IC (essentially an NPN transistor with intergrated resistors). I had to add a pullup resistor too (as seen by the SMT resistor).

I placed polyimide tape over the ground plane to create an isolated work surface. I CA'd the two new parts to the tape. The jumper wires are 40 AWG magnet wire. Most of the soldering work was performed under a magnified light source.

This mod is only shown for those that are curious as to what I did. I do not expect anyone else to be nuts enough to repeat my work. Instead, lean on Plantraco to release a JR compatible Rx. ;)


Regards,
Mr. RC-CAM