This may be old info, but I figured I would post some images of the signals coming out of the receiver interface to the Vex (RX1) input. Looks like the typical serial data stream output from any typical R/C system. The six channels are represented from left to right by the wider pulses dropping down to active low (digital 0), or “negative shift” from sitting normal high (digital 1). They change their width while the others just “push over”, in a format we call Pulse Position Modulation (PPM), unlike what others may have been calling it, Pulse Width Modulation (PWM), and incorrectly so…
The signals were taken off of the white lead, using the red lead as the “reference ground”, with an isolated (floating) scope ground, all while the system was connected and active.
A scope is the way to go as you can’t do much with pulses without one. You don’t need to spend much, check out the USB style from Vellemann and Parallax. I have and use both, in addition to high-end scopes in my work lab.
Looks like the actual receiver output is from the lead red, with respect to ground (the green lead), with a positive shift as active high. You must have a scope with at least 1 Meg Ohm input impedance, otherwise the signal gets loaded and un-stable. I will buffer that output for use later on - serial pulse conversion. The white lead supplies the receiver with +5 volts (regulated) from the Vex contoller. Unsure about the yellow lead - a constant 1.3 volts from the transmitter and the Vex controller, present independant of each other.
Do you use Microchip or Atmel controllers on a regular basis? Spotted an earlier post by you that you might try serial to parallel pulse conversion also. Let me know how the progress goes.
I have some PPM code for the AVR that I’m working on getting working with the VEX system. I have built my own VEX controller out of an AVR and communicate directly to the PC via 2.4ghz RF modules I got from sparkfun.com. I hope soon to have the PPM code working so I can control it directly with the remote as well. Soon as I do that I will be buying a second radio set and crystal pack too.
An oscilloscope, either analog or digital, is one of the most important tools an engineer working on circuitry must have - besides his/her brains, they do come first! The “O-scope” can display AC and DC signals, which are displayed with amplitude (Y-axis) in respect to time (X-axis) and give you the “heartbeat” of a system overall. I have one ready at all times while working on electronic systems, either on the lab bench, or as I do now, plugged into my laptop. I am watching the dis-charge curves (amp-hour rate) of three Vex Ni-Cd packs, plotted against time while I am typing this, with a digital scope and a computerized battery test system. Check out the attached graph (very low resolution though…).
Best of luck with your interest in science & technology.
I’m not sure who “we” are; but generally when information is encoded in the width of a pulse, I call that a Pulse Width Modulation scheme. Are you asserting that the Vex receiver detects a reference point in the incoming signal and then measures from that reference point to the position of a pulse in order to determine the user-controller value that has been encoded (modulated) into the transmitted signal?
Of course it is true that changing the width of a pulse does change the “position” of the successive pulses in any given output cycle, because there is no fixed time separation between the start of any one pulse and the start of the next. Is this the reason that you say modulatiion should be called PPM instead of PWM?
Regardless of this side effect (successive pulse are pushed to the right) of varying the width of a pulse, My hunch is that in order to determine what is encoded into a pulse, the receiver is directly measuring the width of each pulse, not the position of the pulse that follows the one being measured. Hence, my hunch that PWM is the more precise term to use.
Am I overlooking something, or do you know more about the guts of the receiver than I?
